Dissolved air flotation system and methods for biological nutrient removal

ABSTRACT

A wastewater treatment system including a contact tank, a dissolved air flotation unit, a fermentation unit, and a biological treatment unit is disclosed. A method of retrofitting a wastewater treatment system by arranging the wastewater treatment system such that floated biosolids are fermented in an anerobic environment and fluidly connecting the biological treatment unit to receive at least a portion of the fermented solids is also disclosed. The method optionally includes providing a fermentation unit and fluidly connecting the fermentation unit to a biological treatment unit. A method of treating wastewater including combining the wastewater with activated sludge, floating biosolids from the activated wastewater, fermenting the floated biosolids, and biologically treating the effluent with the fermented solids is also disclosed. A method of facilitating delivery of soluble organic carbon to a biological treatment unit is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119 of U.S. patentapplication Ser. No. 62/641,721, titled “Enhancement to CaptivatorSystem for Biological Nutrient Removal,” filed on Mar. 12, 2018, whichis incorporated herein by reference in its entirety for all purposes.

FIELD OF TECHNOLOGY

Aspects and embodiments disclosed herein are directed toward systems andmethods for the treatment of wastewater.

SUMMARY

In accordance with one aspect, there is provided a wastewater treatmentsystem. The wastewater treatment system may comprise a contact tank, adissolved air flotation unit, a fermentation unit, and a biologicaltreatment unit. The contact tank may have a first inlet fluidlyconnectable to a source of wastewater to be treated and an outlet. Thecontact tank may be configured to treat the wastewater with activatedsludge to form a first mixed liquor. The dissolved air flotation unitmay have a first inlet fluidly connected to the outlet of the contacttank, a second inlet fluidly connected to a source of gas, a firstfloated solids outlet, and an effluent outlet. The dissolved airflotation unit may be configured to treat the first mixed liquor withthe gas to form the floated solids and the effluent. The fermentationunit may have a first inlet fluidly connected to the first floatedsolids outlet and a first fermented solids outlet. The fermentation unitmay be configured to treat at least a portion of the floated solids toform the fermented solids. The biological treatment unit may have afirst inlet fluidly connected to the effluent outlet, a second inletfluidly connected to the first fermented solids outlet, and an outlet.The biological treatment unit may be configured to treat the effluentwith at least a portion of the fermented solids to form a second mixedliquor.

In some embodiments, the contact tank may have a second inlet and thebiological treatment unit may have a third inlet. The system may furthercomprise a solids-liquid separation unit having an inlet fluidlyconnected to the outlet of the biological treatment unit, a solids-leaneffluent outlet, a first return activated sludge outlet fluidlyconnected to the second inlet of the contact tank, and a second returnactivated sludge outlet fluidly connected to the third inlet of thebiological treatment unit. The solids-liquid separation unit may beconfigured to treat solids from the third mixed liquor to form thesolids-lean effluent and the return activated sludge.

The system may comprise a screen positioned between the first floatedsolids outlet and the first inlet of the fermentation unit. The screenmay be configured to retain non-biological waste solids.

In some embodiments, the dissolved air flotation unit may furthercomprise a second floated solids outlet and the fermentation unit mayfurther comprise a second fermented solids outlet. The system mayfurther comprise an anaerobic digester having a first inlet fluidlyconnected to the second floated solids, a second inlet fluidly connectedto the second fermented solids outlet, and an outlet.

In accordance with certain embodiments, the system may further comprisea first metering valve positioned upstream from the first fermentedsolids outlet and the second fermented solids outlet. The system mayfurther comprise a first controller operatively connected to the firstmetering valve and configured to instruct the first metering valve toselectively dispense the fermented solids to the biological treatmentunit and to the anaerobic digester.

The system may further comprise a screen positioned between the secondfloated solids outlet and the first inlet of the anaerobic digester. Thescreen may be configured to retain non-biological waste solids.

In some embodiments, the fermentation unit may further comprise a thirdfermented solids outlet and the contact tank may further comprise athird inlet fluidly connected to the third fermented solids outlet.

The system may further comprise a screen positioned between the thirdfermented solids outlet and the third inlet of the contact tank. Thescreen may be configured to retain non-biological waste solids.

In accordance with certain embodiments, the contact tank may comprise asecond outlet. The biological treatment unit may comprise an anaerobicregion, an aerated anoxic region, and an aerobic region. The anaerobicregion may have a first inlet fluidly connected to the effluent outletand an outlet. The aerated anoxic region may have a first inlet fluidlyconnected to the second outlet of the contact tank, a second inletfluidly connected to the outlet of the anaerobic region, and an outlet.The aerobic region may have a first inlet fluidly connected to theoutlet of the aerated anoxic region, a second inlet fluidly connected tothe effluent outlet, and an outlet.

In some embodiments, at least one of the anaerobic region and theaerated anoxic region may have an inlet fluidly connected to the firstfermented solids outlet.

The anaerobic region may have a second inlet fluidly connected to thefirst fermented solids outlet and the aerated anoxic region may have athird inlet fluidly connected to the first fermented solids outlet. Thesystem may further comprise a second metering valve positioneddownstream from the first fermented solids outlet. The system mayfurther comprise a second controller operatively connected to the secondmetering valve and configured to instruct the second metering valve toselectively dispense the fermented solids to the anaerobic region and tothe aerated anoxic region.

The biological treatment unit may further comprise a post-anoxic regionhaving a first inlet fluidly connected to the outlet of the aerobicregion, a second inlet fluidly connected to the first fermented solidsoutlet, and an outlet.

In accordance with another aspect, there is provided a method oftreating wastewater. The method may comprise directing the wastewaterinto a contact tank and mixing the wastewater with an activated sludgeto form an activated mixed liquor. The method may comprise directing theactivated mixed liquor into a dissolved air flotation unit andseparating the activated mixed liquor to form a floated biosolids and aneffluent. The method may comprise selectively directing a first portionof the floated biosolids into a fermentation unit and a second portionof the floated biosolids into an anaerobic digester, and fermenting thefirst portion of the floated biosolids in the fermentation unit to formfermented solids. The method may comprise directing the effluent to abiological treatment unit. The method may comprise selectively directinga first portion of the fermented solids into the biological treatmentunit and a second portion of the fermented solids into the anaerobicdigester, and biologically treating the effluent with the first portionof the fermented solids in the biological treatment unit to form abiologically treated mixed liquor.

The method may further comprise directing the biologically treated mixedliquor into a solids-liquid separation unit and separating thebiologically treated mixed liquor to form a solids-lean effluent and theactivated sludge. The method may comprise directing a first portion ofthe activated sludge into the contact tank. The method may comprisedirecting a second portion of the activated sludge into the biologicaltreatment unit.

In accordance with certain aspects, the method may further compriseseparating non-biological waste solids from at least one of the firstportion of the floated biosolids and the second portion of the floatedbiosolids.

The method may comprise directing a third portion of the fermentedsolids into the contact tank.

The method may comprise separating non-biological waste solids from thethird portion of the fermented solids.

In some embodiments, the first portion of the fermented solids may beselectively directed into at least one of an anaerobic region, anaerated anoxic region, and a post-anoxic region of the biologicaltreatment unit.

An amount of the first portion of the fermented solids directed into thebiological treatment unit may be selected responsive to a determinationof a target soluble chemical oxygen demand of the biological treatmentunit.

In some embodiments, treating the effluent with the first portion of thefermented solids may comprise reducing a concentration of dissolvedphosphorous in the effluent when forming the biologically treated mixedliquor.

In accordance with another aspect, there is provided a method ofretrofitting a wastewater treatment system comprising a contact tank, adissolved air flotation unit, and a biological treatment unit. Themethod may comprise arranging the wastewater treatment system such thatat least a portion of a floated biosolids from the dissolved airflotation unit are fermented in an anaerobic environment to form afermented solids. The method may comprise fluidly connecting thebiological treatment unit to receive at least a portion of the fermentedsolids.

In some embodiments, the method may comprise providing a fermentationunit. The method may comprise fluidly connecting the fermentation unitto receive the at least a portion of the floated biosolids. The methodmay comprise fluidly connecting the biological treatment unit to atleast one fermented solids outlet of the fermentation unit.

The method may further comprise fluidly connecting an anaerobic digesterto the at least one fermented solids outlet.

In some embodiments, the system comprises a metering valve positioneddownstream from the at least one fermented solids outlet and acontroller operatively connected to the metering valve. The method maycomprise programming the controller to instruct the metering valve toselectively direct the fermented solids to the biological treatment unitand to the anaerobic digester responsive to a determination of arequired soluble chemical oxygen demand of the biological treatmentunit.

The method may further comprise fluidly connecting the contact tank tothe at least one fermented solids outlet.

In accordance with another aspect, there is provided a method offacilitating delivery of soluble organic carbon to a biologicaltreatment unit in a wastewater treatment system comprising a contacttank, a dissolved air flotation unit, a biological treatment unit, andan anaerobic digester. The method may comprise fluidly connecting thecontact tank to a source of the wastewater to be treated. The method maycomprise instructing a user to determine a target soluble chemicaloxygen demand of the biological treatment unit. The method may compriseinstructing the user to operate the dissolved air flotation unit togenerate floated biosolids. The method may comprise instructing the userto selectively direct a first portion of the fermented solids to thebiological treatment unit and a second portion of the fermented solidsto the anaerobic digester responsive to a measurement of the solublechemical oxygen demand of the biological treatment unit being outsidetolerance of the target soluble chemical oxygen demand.

In some embodiments, the method may comprise providing a fermentationunit configured to ferment the at least a portion of the floatedbiosolids and produce the fermented solids. The method may comprisefluidly connecting the fermentation unit to receive at least a portionof the floated biosolids. The method may comprise fluidly connecting thebiological treatment unit and the anaerobic digester to at least onefermented solids outlet of the fermentation unit.

The method may further comprise providing a controller. The method mayfurther comprise programming the controller to selectively direct thefirst portion of the fermented solids to the biological treatment unitand the second portion of the fermented solids to the anaerobicdigester, in an amount sufficient to restore the soluble chemical oxygendemand to be within tolerance of the target soluble chemical oxygendemand.

In some embodiments, the method may further comprise instructing theuser to measure a concentration of dissolved phosphorous in wastewatertreated by the system and instructing the user to selectively direct thefirst portion of the fermented solids to the biological treatment unitand the second portion of the fermented solids to the anaerobic digesterresponsive to the measurement of the concentration of the dissolvedphosphorous being outside tolerance of a target concentration of thedissolved phosphorous in the treated wastewater.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a block flow diagram of a wastewater treatment system inaccordance with an embodiment;

FIG. 2 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 3 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 4 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 5 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 6 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 7 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 8 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 9 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 10 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 11 illustrates a first set of results of a test of a system inaccordance with an embodiment;

FIG. 12 illustrates a second set of results of a test of a system inaccordance with an embodiment;

FIG. 13 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 14 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 15 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 16 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment; and

FIG. 17 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

As the term is used herein, an “upstream” unit operation refers to afirst unit operation which is performed upon a fluid undergoingtreatment prior to a second unit operation. Similarly, an “upstream”treatment vessel or portion thereof refers to a first treatment vesselor portion thereof in which a first unit operation is performed prior toa second unit operation performed in a second treatment vessel orportion thereof. A “downstream” unit operation refers to a second unitoperation which is performed upon a fluid undergoing treatmentsubsequent to a first unit operation. Similarly, a “downstream”treatment vessel or portion thereof refers to a second treatment vesselor portion thereof in which a second unit operation is performedsubsequent to a first unit operation performed in a first treatmentvessel or portion thereof. An upstream unit operation and/or treatmentvessel having an outlet in “direct fluid communication” with an inlet ofa downstream unit operation and/or treatment vessel directs materialoutput from the outlet of the upstream unit operation and/or treatmentvessel into the inlet of the downstream unit operation and/or treatmentvessel without any intervening operations performed on the material. Afirst unit operation and/or treatment vessel described herein as beingin fluid communication with a second unit operation and/or treatmentvessel should be understood as being in direct fluid communication withthe second unit operation and/or treatment vessel unless explicitlydescribed as otherwise. Conduits which provide fluid communicationbetween a first and a second unit operation and/or treatment vessel areto be understood as providing direct fluid communication between thefirst and second unit operation and/or treatment vessel unlessexplicitly described as otherwise.

Various unit operations and/or treatment vessels disclosed hereinseparate fluid and/or sludge into a solids-rich portion and asolids-lean portion wherein the solid-lean potion has a lowerconcentration of solids than the solids-rich portion. As the term isused herein, an “effluent” of a unit operation and/or treatment vesselrefers to the solids-lean portion of the separated fluid and/or sludge.“Recycle” of material refers to directing material from an outlet of adownstream unit operation and/or treatment vessel to an inlet of a unitoperation and/or treatment vessel upstream of the downstream unitoperation and/or treatment vessel.

U.S. Pat. Nos. 8,808,544 and 10,131,550, titled “ContactStabilization/Prime Float Hybrid” and “Enhanced Biosorption ofWastewater Organics Using Dissolved Air Flotation with Solids Recycle,”respectively, are incorporated herein by reference in their entiretiesfor all purposes.

Aspects and embodiments of the present invention are directed towardsystems and methods for treating wastewater. As used herein the term“wastewater” includes, for example, municipal wastewater, industrialwastewater, agricultural wastewater, and any other form of liquid to betreated containing undesired contaminants. Aspects and embodiments ofthe present invention may be utilized for primary wastewater treatment,secondary wastewater treatment, or both. Aspects and embodiments of thepresent invention may remove sufficient contaminants from wastewater toproduce product water that may be used for, for example, irrigationwater, potable water, cooling water, boiler tank water, or for otherpurposes.

In some embodiments, the apparatus and methods disclosed herein provideadvantages with regard to, for example, capital costs, operationalcosts, and environmental-friendliness as compared to conventionalbiological wastewater treatment systems. In some embodiments a dissolvedair flotation system is included in a main stream of wastewater enteringa biological wastewater treatment system. The dissolved air floatationsystem may remove a significant amount of biological oxygen demand, forexample, particulate biological oxygen demand, from wastewater prior tothe wastewater entering the biological treatment portion of thewastewater treatment system. This provides for a reduction in the sizeof the biological treatment portion of the wastewater treatment systemfor a given wastewater stream as compared to a conventional wastewatertreatment system and a commensurate reduced capital cost for the overallsystem. Utilization of the dissolved air flotation system also reducesthe requirement for aeration in the biological treatment portion of thetreatment system to effect oxidation of the biological oxygen demand ofthe wastewater, reducing operating costs. The amount of waste sludgegenerated by the biological treatment portion of the treatment system isalso reduced, reducing the amount of waste which would need to bedisposed of or otherwise further treated. The material removed from thewastewater in the dissolved air flotation system may be utilized toproduce energy, for example, in the form of biogas in a downstreamanaerobic digestion system. The biogas may be used to provide salableenergy through combustion or through use in, for example, fuel cells.

In accordance with an embodiment of the present invention there isprovided a method of facilitating increased operating efficiency of awastewater treatment system. The method comprises configuring adissolved air flotation (DAF) unit in a wastewater treatment system influid communication between a contact tank and a biological treatmentunit to remove solids from a portion of a first mixed liquor output fromthe contact tank prior to the portion of the first mixed liquor enteringthe biological treatment unit and to recycle at least a portion of thesolids to the contact tank, the recycle of the at least a portion of thesolids to the contact tank reducing an amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of recycling the atleast a portion of the solids to the contact tank,.

In some embodiments, greater than 50% of the solids are recycled fromthe DAF unit to the contact tank.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to increase biogasproduction of an anaerobic digester of the wastewater treatment systemhaving an inlet in fluid communication with an outlet of the DAF unit,at least a second portion of the solids removed in the DAF unit beingdirected into the anaerobic digester.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to reduce the energyconsumption of the wastewater treatment system.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet, a second inlet, and anoutlet and a dissolved air flotation tank having an inlet in fluidcommunication with the outlet of the contact tank, a first outlet, and asecond outlet. The wastewater treatment system further comprises anaerated anoxic tank having a first inlet in fluid communication with theoutlet of the contact tank, a second inlet, and an outlet and aerobictank having a first inlet in fluid communication with the outlet of theaerated anoxic tank, a second inlet in fluid communication with thefirst outlet of the dissolved air flotation tank, and an outlet. Thewastewater treatment system further comprises a clarifier having aninlet in fluid communication with the outlet of the aerobic tank and anoutlet in fluid communication with the second inlet of the contact tankand with the second inlet of the aerated anoxic tank.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank, mixing the wastewaterwith activated sludge in the contact tank to form a mixed liquor,transporting a first portion of the mixed liquor to a dissolved airflotation tank, separating the first portion of the mixed liquor in thedissolved air flotation tank to form a dissolved air flotation tankeffluent and waste biosolids, transporting a second portion of the mixedliquor to an aerated anoxic treatment tank, biologically treating thesecond portion of the mixed liquor in the aerated anoxic treatment tankto form an anoxic mixed liquor, transporting the anoxic mixed liquor toan aerobic treatment tank, transporting the dissolved air flotation tankeffluent to the aerobic treatment tank, biologically treating the anoxicmixed liquor and the dissolved air flotation tank effluent in theaerobic treatment tank to form an aerobic mixed liquor, transporting theaerobic mixed liquor to a clarifier, separating the aerobic mixed liquorin the clarifier to form a clarified effluent and a return activatedsludge, recycling a first portion of the return activated sludge to thecontact tank, and recycling a second portion of the return activatedsludge to the aerated anoxic treatment tank.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet configured to receivewastewater to be treated, a second inlet, and an outlet. The contacttank is configured to mix the wastewater to be treated with activatedsludge to form a first mixed liquor. The system further comprises a DAFunit having an inlet in fluid communication with the outlet of thecontact tank, a solids outlet, a DAF unit effluent outlet, and a gasinlet. The gas inlet is configured to introduce gas into the DAF unit tofacilitate the flotation of suspended matter from the first mixed liquorand the removal of the suspended matter from the DAF unit. The solidsoutlet is in fluid communication with the first inlet of the contacttank and configured to transfer at least a portion of the suspendedmatter from the DAF unit to the first inlet of the contact tank. Thesystem further comprises a biological treatment unit having a firstinlet in fluid communication with the outlet of the contact tank, asecond inlet, a third inlet in fluid communication with the DAF uniteffluent outlet, and an outlet. The biological treatment unit isconfigured to biologically break down organic components of the firstmixed liquor and of an effluent from the DAF unit to form a second mixedliquor. The system further comprises a clarifier having an inlet influid communication with the outlet of the biological treatment unit, aneffluent outlet, and a return activated sludge outlet in fluidcommunication with the second inlet of the contact tank and with thesecond inlet of the biological treatment unit. The clarifier isconfigured to output a clarified effluent through the effluent outletand a return activated sludge though the return activated sludge outlet.

In accordance with some aspects of the wastewater treatment system, thebiological treatment unit includes an aerated anoxic region having afirst inlet in fluid communication with the outlet of the contact tank,a second inlet, and an outlet and an aerobic region having a first inletin fluid communication with the outlet of the aerated anoxic region, asecond inlet in fluid communication with the DAF unit effluent outlet,and an outlet.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are included in a sametreatment tank.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are separated by apartition.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region is included in a first treatment tank and theaerobic region is included in a second treatment tank distinct from thefirst treatment tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system comprises a first sub-system including thecontact tank and the DAF unit which is physically separated from asecond sub-system including the biological treatment unit and theclarifier.

In accordance with some aspects of the wastewater treatment system, thecontact tank and the aerated anoxic region are included in a same tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises an anaerobic digesterhaving an inlet in fluid communication with the solids outlet of the DAFunit and an outlet.

In accordance with some aspects of the wastewater treatment system, theoutlet of the anaerobic digester is in fluid communication with at leastone of the contact tank and the biological treatment unit.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a primary clarifier havingan inlet in fluid communication with a source of the wastewater to betreated and a solids-lean outlet in fluid communication with the contacttank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a thickener having aninlet in fluid communication with a solids-rich outlet of the primaryclarifier and an outlet in fluid communication with the anaerobicdigester.

In accordance with some aspects of the wastewater treatment system, theprimary clarifier further comprises a solids-rich outlet in fluidcommunication with the DAF unit.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank including an activatedsludge, mixing the wastewater with activated sludge in the contact tankto form a mixed liquor, and directing a first portion of the mixedliquor to a DAF unit. The method further comprises separating the firstportion of the mixed liquor in the DAF unit to form a DAF unit effluentand separated biosolids, directing at least a portion of the separatedbiosolids from the DAF unit to the contact tank, directing a secondportion of the mixed liquor to a biological treatment unit, directingthe DAF unit effluent to the biological treatment unit, biologicallytreating the mixed liquor and the DAF unit effluent in the biologicaltreatment unit to form a biologically treated mixed liquor, anddirecting the biologically treated mixed liquor to a clarifier. Themethod further comprises separating the biologically treated mixedliquor in the clarifier to form a clarified effluent and a returnactivated sludge, recycling a first portion of the return activatedsludge to the contact tank, recycling a second portion of the returnactivated sludge to the biological treatment unit, and directing theclarified effluent to a treated wastewater outlet.

In accordance with some aspects of the method of treating wastewaterwherein the biological treatment unit includes an aerated anoxictreatment unit and an aerobic treatment unit, the method furthercomprises directing the second portion of the mixed liquor to theaerated anoxic treatment unit, treating the second portion of the mixedliquor in the aerated anoxic treatment unit to form an anoxic mixedliquor, directing the anoxic mixed liquor to the aerobic treatment unit,directing the DAF unit effluent to the aerobic treatment unit, treatingthe anoxic mixed liquor and the DAF unit effluent in the aerobictreatment tank to form an aerobic mixed liquor, directing the aerobicmixed liquor to the clarifier, separating the aerobic mixed liquor inthe clarifier to form the clarified effluent and the return activatedsludge, and recycling the second portion of the return activated sludgeto the aerated anoxic treatment unit.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge and the second portionof the return activated sludge comprise about 100% of all returnactivated sludge formed in the clarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge comprises between about10% and about 20% of all return activated sludge recycled from theclarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the mixed liquor comprises between about one thirdand about two thirds of all mixed liquor formed in the contact tank.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 60% and about 100% of suspendedsolids in the first portion of the mixed liquor from the first portionof the mixed liquor.

In accordance with some aspects of the method of treating wastewater, anamount of suspended solids removed in the DAF unit is adjusted basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 40% and about 80% of biologicaloxygen demand in the first portion of the mixed liquor from the firstportion of the mixed liquor.

In accordance with some aspects of the method of treating wastewater,the method further comprises treating at least a portion of the wastebiosolids in an anaerobic digester to produce an anaerobically digestedsludge.

In accordance with some aspects of the method of treating wastewater,the method further comprises recycling at least a portion of theanaerobically digested sludge to at least one of the contact tank andthe biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the method further comprises separating the water to be treated into asolids-lean portion and a solids-rich portion, directing the solids-richportion into a thickener to produce a solids-rich output and asolids-lean effluent, directing the solids-lean portion into the contacttank, directing the solids-rich output from the thickener into theanaerobic digester, and directing the solids-lean effluent of thethickener into the contact tank.

In accordance with another embodiment of the present invention there isprovided method of facilitating increased operating efficiency of awastewater treatment system. The method comprises providing a DAF unitin a wastewater treatment system in fluid communication between acontact tank and a biological treatment unit, the DAF unit configured toremove solids from a portion of a first mixed liquor output from thecontact tank prior to the portion of the first mixed liquor entering thebiological treatment unit and to recycle at least a portion of thesolids to the contact tank, reducing the amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of the DAF unit,and providing for a solids-liquid separation unit in fluid communicationdownstream of the biological treatment unit to recycle a returnactivated sludge formed from a mixed liquor output from the biologicaltreatment unit to the contact tank.

In accordance with some aspects, the method further comprises providingfor between about 10% and about 20% of the return activated sludgeformed to be recycled to the contact tank.

In accordance with some aspects, the method further comprises adjustingan amount of return activated sludge recycled to the contact tank basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects, the method further comprises providingan anaerobic digester having an inlet in fluid communication with anoutlet of the DAF unit and an outlet in fluid communication with atleast one of an inlet of the contact tank and an inlet of the biologicaltreatment unit.

A first embodiment, indicated generally at 100, is illustrated inFIG. 1. Wastewater from a source of wastewater 105 is directed into acontact tank 110 through an inlet of the contact tank. In the contacttank 110, the wastewater is mixed with activated sludge recycled througha conduit 175 from a downstream biological treatment process describedbelow. In some embodiments, the contact tank 110 is aerated tofacilitate mixing of the wastewater and the activated sludge. Theaeration gas may be an oxygen containing gas, for example, air. Thecontact tank 110 may be provided with sufficient oxygen such thataerobic conditions are maintained in at least a portion of the contacttank 110. For example, the contact tank 110 may be aerated. In otherembodiments, the contact tank 110 may not be aerated. Suspended anddissolved solids in the wastewater, including oxidizable biologicalmaterials (referred to herein as Biological Oxygen Demand, or BOD), areadsorbed/absorbed into the activated sludge in the contact tank, forminga first mixed liquor. A portion of the BOD may also be oxidized in thecontact tank 110. The residence time of the wastewater in the contacttank may be sufficient for the majority of the BOD to beadsorbed/absorbed by the activated sludge, but no so long as for asignificant amount of oxidation of the BOD to occur. In someembodiments, for example, less than about 10% of the BOD entering thecontact tank 110 is oxidized in the contact tank. The residence time ofthe wastewater in the contact tank is in some embodiments from about 30minutes to about two hours, and in some embodiments, from about 45minutes to about one hour. The residence time may be adjusted dependingupon factors such as the BOD of the influent wastewater. A wastewaterwith a higher BOD may require longer treatment in the contact tank 110than wastewater with a lower BOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a dissolved air flotation (DAF) system 120 through conduit114. The DAF system may include a vessel, tank, or other open or closedcontainment unit configured to perform a dissolved air flotationoperation as described below. For the sake of simplicity a dissolved airflotation system will be referred to herein as a “DAF unit.” The DAFunit 120 may function as both a thickener and a clarifier. FIG. 1illustrates two DAF units 120 operating in parallel, however, otherembodiments may have a single DAF unit or more than two DAF units.Providing multiple DAF units provides for the system to continueoperation if one of the DAF units is taken out of service for cleaningor maintenance.

Before entering the DAF unit(s), air or another gas may be dissolved inthe first mixed liquor under pressure. The pressure may be released asthe first mixed liquor enters the DAF unit(s) 120, resulting in the gascoming out of solution and creating bubbles in the mixed liquor. In someembodiments, instead of dissolving gas into the first mixed liquor, afluid, for example, water having a gas, for example, air, dissolvedtherein, is introduced into the DAF unit(s) 120 with the first mixedliquor. Upon the mixing of the first mixed liquor and the gas-containingfluid, bubbles are produced. The bubbles formed in the DAF unit(s) 120adhere to suspended matter in the first mixed liquor, causing thesuspended matter to float to the surface of the liquid in the DAFunit(s) 120, where it may be removed by, for example, a skimmer.

In some embodiments, the first mixed liquor is dosed with a coagulant,for example, ferric chloride or aluminum sulfate prior to or afterintroduction into the DAF unit(s) 120. The coagulant facilitatesflocculation of suspended matter in the first mixed liquor.

In the DAF unit(s) 120 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process. At least a portion of any oil that maybe present in the first mixed liquor may also be removed in the DAFunit(s) 120. In some embodiments, a majority, for example, about 60% ormore, about 75% or more, or about 90% or more of the suspended solids inthe first mixed liquor introduced into the DAF unit(s) 120 is removedand about 40% or more, for example, about 50% or more or about 75% ormore of the BOD is removed. Removal of the BOD may include enmeshmentand adsorption in the first mixed liquor and/or oxidation of the BOD andthe formation of reaction products such as carbon dioxide and water. Inother embodiments, up to about 100% of the suspended solids is removedin the DAF unit(s) 120 and a majority, for example, up to about 80% ofthe BOD is removed.

In some embodiments, suspended solids removed in the DAF unit(s) 120 aresent out of the system as waste solids through a conduit 125. Thesewaste solids may be disposed of, or in some embodiments, may be treatedin a downstream process, for example, an anaerobic digestion process oranaerobic membrane bioreactor to produce useful products, for example,biogas and/or usable product water.

In other embodiments, at least a portion of the suspended solids removedin the DAF unit(s) 120 are recycled back to the contact tank 110 throughconduits 125 and 126. Conduit 126 may branch off of conduit 125 asillustrated, or may be connected to a third outlet of the DAF unit(s)120, in which case suspended solids removed in the DAF unit(s) 120 arerecycled back to the contact tank 110 through conduit 126 only. Theamount of solids recycled from DAF unit(s) 120 to the contact tank 110may range from about 1% to about 100% of a total amount of solidsremoved from the first mixed liquor in the DAF unit(s) 120. The amountof solids recycled from DAF unit(s) 120 to the contact tank 110 may be amajority of a total amount of solids removed from the first mixed liquorin the DAF unit(s) 120, for example, greater than about 50%, betweenabout 50% and about 95%, or between about 60% and about 80% of the totalamount of solids removed from the first mixed liquor in the DAF unit(s)120.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110is counter to the conventional operation of wastewater treatment systemsincluding DAF units. Typically, DAF units are utilized in wastewatertreatment systems to remove solids from the wastewater, thus reducingthe need for biological treatment of these removed solids and reducingthe energy requirements of the wastewater treatment system by, forexample, reducing the amount of air needed to be supplied to an aeratedbiological treatment vessel to oxidize the removed solids. It is counterto conventional operation of wastewater treatment systems tore-introduce floated solids separated from mixed liquor from a contacttank in DAF unit(s) back to the contact tank. Typically, after solidsare separated from mixed liquor from a contact tank in DAF unit(s),reintroducing the separated solids into mixed liquor in the contact tankand forcing the solids to go through the same separation process in theDAF unit(s) would reduce the efficiency of the system. Such a solidsrecycle from DAF unit(s) to a contact tank directly upstream of the DAFunit(s) would cause a need for a greater amount of contact tank capacityand a greater amount of DAF unit capacity. Such a solids recycle fromDAF unit(s) to a contact tank directly upstream of the DAF unit(s) wouldalso require more air flow to the DAF unit(s) to remove the recycledsolids from the mixed liquor in addition to any solids that would bepresent in the absence of the solids recycle. It has been discovered,however, that benefits may be achieved by the counterintuitivere-introduction of solids removed in DAF unit(s) back into the contacttank of a wastewater treatment system from which mixed liquor issupplied to the DAF unit(s).

For example, by recycling the solids removed by the DAF unit(s) 120 tothe contact tank 110, the amount of total suspended solids (TSS) in thecontact tank 110 may be increased as compared to methods not including arecycle of solids from the DAF unit(s) 120 to the contact tank 110. Theincreased TSS level in the contact tank 110 may provide for additionalsoluble BOD to be adsorbed in the contact tank 110 as compared to acontact tank 110 having a lower level of TSS. In some embodiments, adesirable TSS level in the contact tank 110 may be between about 1,200mg/L and about 3,500 mg/L.

The removal of the additional soluble BOD in the contact tank 110 due tothe higher TSS level in the contact tank 110, resulting from the recycleof solids from the DAF unit(s) 120 to the contact tank 110, provides forthe removal of this additional BOD as solids in the DAF unit(s) 120. Theadditional BOD removed as solids in the DAF unit(s) 120 may be directedto an anaerobic digester (for example, anaerobic digester 490illustrated in FIG. 4) rather than an aerated biological treatment unit(for example, biological treatment unit 130), thus reducing the need foraeration power in the biological treatment unit and increasing theamount of biogas that could be produced in the anaerobic digester.

When supplied with recycled solids from the DAF unit(s) 120, the contacttank 110 may have a hydraulic retention time (HRT) of between about 15minutes and about one hour and a solids retention time (SRT) of betweenabout 0.5 days and about two days to effectively adsorb soluble BOD. Inother embodiments, the SRT in the contact tank may be between about 0.2and about 0.4 days. When the contact tank 110 includes TSS in a range ofbetween about 1,200 mg/L and about 3,500 mg/L, a sludge age (SRT) in thecontact tank may range from about one to about two days.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110provides for the contact tank 110 to function as a high rate activatedsludge system while the DAF unit(s) 120 function a solids-liquidseparator. Recycling solids removed in the DAF unit(s) 120 to thecontact tank 110 provides for greater oxidation of BOD in the contacttank 110 than in systems where solids removed from the DAF unit(s) 120are not recycled to the contact tank because the solids recycled to thecontact tank includes living bacteria capable of oxidizing BOD. Forexample, in systems and methods where solids removed in the DAF unit(s)120 are recycled to the contact tank 110, oxidation of greater thanabout 10% of the BOD in wastewater influent to the contact tank 110 maybe oxidized in the contact tank 110. Recycling solids removed in the DAFunit(s) 120 to the contact tank 110 may thus reduce the amount of BODthat needs to be treated in downstream unit operations, for example, inthe biological treatment unit 130 discussed below, thus reducing thepower requirements for the downstream unit operations. The SRT of thecontact tank 110 may be adjusted to optimize BOD removal of particulate,colloidal, and soluble BOD fractions.

Effluent from the DAF unit(s) 120 is directed through conduit 124 intothe biological treatment unit 130, which may include one or moretreatment tanks. In some embodiments, the biological treatment unit 130may comprise a contact stabilization vessel. A portion of the effluentmay be recycled (recycle system not shown in FIG. 1) to supply gasbubbles to the DAF unit(s) 120. A gas may be dissolved into the recycledportion of effluent, which is then directed back into the DAF unit(s)120 and mixed with influent first mixed liquor.

A second portion of the first mixed liquor formed in the contact tank isdirected into the biological treatment unit 130 through a conduit 115.In some embodiments, about a half of the first mixed liquor formed inthe contact tank is directed into the DAF unit(s) 120 and about a halfof the first mixed liquor formed in the contact tank is directed throughthe conduit 115 into the biological treatment unit 130. In otherembodiments, between about one third and two thirds of the first mixedliquor formed in the contact tank is directed into the DAF unit(s) 120and the remainder of the first mixed liquor formed in the contact tankis directed through the conduit 115 into the biological treatment unit130. The amount of the first mixed liquor directed into the DAF unit(s)120 as opposed to the biological treatment unit 130 may be varied basedupon such factors as the concentration of the first mixed liquor and theeffectiveness of the first mixed liquor at enmeshing BOD in the contacttank 110.

For example, if it was desired to remove a greater rather than a lesseramount of solids in the DAF unit(s) 120, a greater fraction of the firstmixed liquor from the contact tank would be directed to the DAF unit(s)120 when the first mixed liquor had a lower rather than a higherconcentration of solids. Similarly, if it was desired to remove agreater rather than a lesser amount of BOD in the DAF unit(s) 120, agreater fraction of the first mixed liquor from the contact tank wouldbe directed to the DAF unit(s) 120 when the first mixed liquor had alesser rather than a greater effectiveness at enmeshing BOD in thecontact tank.

In the biological treatment unit 130, the effluent from the DAF unit(s)120 and the first mixed liquor formed in the contact tank 110 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 130 includes oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 130 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 130 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 130. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor, and the biologicaltreatment unit 130, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 130. The residence time of the second mixedliquor in the biological treatment unit 130 may be sufficient to oxidizesubstantially all BOD in the second mixed liquor. Residence time for thesecond mixed liquid in the biological treatment unit 130 may be fromabout three to about eight hours. This residence time may be increasedif the influent wastewater to be treated and/or the second mixed liquorcontains a high level of BOD or decreased if the influent wastewater tobe treated and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 130is directed through a conduit 135 into a separation apparatus, which mayinclude, for example, a clarifier 140, a gravity separation apparatus,and/or another form of separation apparatus. Effluent from the clarifier140 may be directed to a product water outlet through a conduit 145 orbe sent on for further treatment. Activated sludge separated fromeffluent in the clarifier may be recycled back upstream to a wastewaterinlet of the system, the source of wastewater, the contact tank 110through conduits 155 and 175, and/or the biological treatment unit 130through conduits 155 and 165. In some embodiments 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments between about 10% and about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through the conduit175 and between about 80% and 90% of the recycled sludge is directedinto the biological treatment unit 130 through the conduit 165. Theamount of recycled sludge directed to the wastewater inlet and contacttank through the conduit 175 may be set at a higher end of this rangewhen the incoming wastewater has a high level of BOD and/or when therecycled sludge is less rather than more effective at enmeshing BOD inthe contact tank 110. The amount of recycled sludge directed to thewastewater inlet and contact tank through the conduit 175 may be set ata lower end of this range when the incoming wastewater has a low levelof BOD and/or when the recycled sludge is more rather than lesseffective at enmeshing BOD in the contact tank 110.

The amount of activated sludge separated in the clarifier 140 which isrecycled to the contact tank 110 and/or biological treatment unit 130may also be adjusted based on a fraction of the first mixed liquor fromthe contact tank 110 which is directed to the DAF unit(s) 120, theamount of activated sludge which is removed in the DAF units(s) 120,and/or the amount of activated sludge removed in the DAF units(s) 120which is recycled to the contact tank 110. The amount of activatedsludge which is recycled to the contact tank 110 and/or biologicaltreatment unit 130 may be an amount equal to or greater than an amountrequired to maintain a desired population of bacteria in the biologicaltreatment unit 130 to perform biological treatment of the second mixedliquor within a desired timeframe and/or to protect against depletion ofthe bacterial population in the event of temporary disruptions in theoperation of the treatment system. For example, the amounts of activatedsludge which is recycled to the contact tank 110 or biological treatmentunit 130 may be set such that sufficient bacteria containing solids arepresent in the biological treatment unit 130 to result in a SRT ofbetween about one and about 10 days in the biological treatment unit130. Similarly, an amount or fraction of the first mixed liquor directedinto the DAF unit(s) 120 may be adjusted based on the amount ofactivated sludge recycled from the clarifier 140, the efficiency ofremoval of solids in the DAF unit(s) 120 and/or the concentration of oneor more types of bacteria in the biological treatment unit 130 to, forexample, establish or maintain a desired population of bacteria in thebiological treatment unit 130.

In the embodiment illustrated in FIG. 1, and in the additionalembodiments described below, it should be understood that the variousconduits illustrated may be provided with, for example, pumps, valves,sensors, and control systems as needed to control the flow of fluidstherethrough. These control elements are not illustrated in the figuresfor the sake of simplicity.

In another embodiment, indicated generally at 200 in FIG. 2, thebiological treatment unit 130 includes an aerobic region 150 and anaerated anoxic region 160. The aerobic region 150 is in fluidcommunication downstream of the aerated anoxic region 160 and receivesbiologically treated anoxic mixed liquor from the aerated anoxic region.In some embodiments, the aerobic region 150 may be formed in a samevessel or tank as the aerated anoxic region 160 and separated therefromby a partition or weir 195. In other embodiments, the aerobic region 150may be physically separate from the aerated anoxic region 160. Forexample, the aerobic region 150 and the aerated anoxic region 160 mayoccupy distinct vessels or tanks or may be otherwise separated from oneanother. In further embodiments the contact tank 110 may be combinedwith the aerated anoxic region 160 in the same tank.

In the system of FIG. 2 effluent from the DAF unit(s) 120 is directedinto the aerobic region 150 without first passing through the aeratedanoxic region 160. In other embodiments, the effluent from the DAFunit(s) 120 may be introduced into the aerated anoxic region 160 andthen directed into the aerobic region 150.

Another embodiment, indicated generally at 300, is illustrated in FIG.3. In this embodiment, the wastewater treatment system 300 is brokeninto two separate but interconnected subsystems, one subsystem 300Aincluding a contact tank 210 and DAF unit(s) 220, and a second subsystem300B including a biological treatment unit 230 and a separationapparatus 240. In the first subsystem 300A influent wastewater from asource of wastewater 205A is directed into the contact tank 210. In thecontact tank, the wastewater is mixed with activated sludge recycledthrough a conduit 275 from a biological treatment process included insubsystem 300B described below. In some embodiments, the contact tank210 is aerated to facilitate mixing of the wastewater and the activatedsludge. Suspended and dissolved solids in the wastewater areadsorbed/absorbed into the activated sludge in the contact tank 210,forming a first mixed liquor. A portion of the BOD in the influentwastewater may be oxidized in the contact tank 210. The residence timeof the wastewater in the contact tank may be sufficient for the majorityof the BOD to be adsorbed/absorbed by the activated sludge, but no solong as for a significant amount of oxidation of the BOD to occur. Insome embodiments, for example, less than about 10% of the BOD enteringthe contact tank 210 is oxidized in the contact tank. The residence timeof the wastewater in the contact tank is in some embodiments from about30 minutes to about two hours, and in some embodiments, from about 45minutes to about one hour. The residence time may be adjusted dependingupon factors such as the BOD of the influent wastewater. A wastewaterwith a higher BOD may require longer treatment in the contact tank 210than wastewater with a lower BOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a DAF unit 220 through conduit 214. FIG. 3 illustrated twoDAF units 220 operating in parallel, however other embodiments may havea single DAF unit or more than two DAF units. Providing multiple DAFunits provides for the system to continue operation if one of the DAFunits is taken out of service for cleaning or maintenance. A secondportion of the first mixed liquor formed in the contact tank is directedinto the biological treatment unit 230 in the second subsystem 300Bthrough a conduit 215. In some embodiments, about a half of the firstmixed liquor formed in the contact tank is directed into the DAF unit(s)220 and about a half of the first mixed liquor formed in the contacttank is directed through the conduit 215 into the biological treatmentunit 230. In other embodiments, between about one third and two thirdsof the first mixed liquor formed in the contact tank is directed intothe DAF unit(s) 220 and the remainder of the first mixed liquor formedin the contact tank is directed through the conduit 215 into thebiological treatment unit 230. The amount of the first mixed liquordirected into the DAF unit(s) 220 as opposed to the biological treatmentunit 230 may be varied based upon such factors as the concentration ofthe first mixed liquor and the effectiveness of the first mixed liquorat enmeshing BOD in the contact tank 210.

In the DAF unit(s) 220 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process such as that described above withreference to DAF unit(s) 120. The removed suspended solids may be sentout of the system as waste solids through a waste conduit 225. Thesewaste solids may be disposed of or treated in a downstream process, forexample, an anaerobic digestion process or anaerobic membrane bioreactorto produce biogas and/or usable product water. Effluent from the DAFunit(s) 220 is directed to an outlet 224 from which it may be used asproduct water or sent on for further treatment.

In some embodiments, a portion of the suspended solids removed from thefirst mixed liquor in the DAF unit(s) 220 may be recycled to the contacttank 210 through conduits 225 and 226 in a similar manner as the recycleof suspended solids removed in the DAF unit(s) 120 to the contact tank110 described above with reference to FIG. 1.

In the second subsystem 300B, influent wastewater from a source ofwastewater 205B is introduced into the biological treatment unit 230.The source of wastewater 205B may be the same as or different from thesource of wastewater 205A. In the biological treatment unit 230 thewastewater and the first mixed liquor formed in the contact tank 210 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 230 may include oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 230 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 230 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 230. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor and the biologicaltreatment unit 230, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 230.

Residence time for the second mixed liquid in the biological treatmenttank 230 may be from about three to about eight hours. This residencetime may be increased if the influent wastewater to be treated and/orthe second mixed liquor contains a high level of BOD or decreased if thewastewater and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 230is directed through a conduit 235 into a separation apparatus, which mayinclude, for example, a clarifier 240. Effluent from the clarifier 240may be directed to a product water outlet through a conduit 245 or besent on for further treatment. Activated sludge separated from effluentin the clarifier may be recycled back upstream to the biologicaltreatment unit 230 and/or to the contact tank 210 in subsystem 300Athrough a conduit 255. In some embodiments about 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments from about 10% to about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through a conduit 275and from about 80% to about 90% of the recycled sludge is directed intothe biological treatment unit 230 through a conduit 265.

Utilizing DAF units as described above in a wastewater treatment systemprovides several advantages over similar wastewater treatment systemsoperated without DAF units. Because the DAF units remove a significantportion of suspended solids from influent wastewater without the needfor oxidation of these solids, the size of other components of thesystem may be reduced, resulting in a lower capital cost for the system.For example, primary clarifiers may be omitted from the wastewatertreatment system. Due to the reduced amount of oxidized solids to beremoved from the system, a final clarifier, such as the clarifier 140,may be reduced in size, in some embodiments by about 50%. Because alower amount of BOD enters the biological treatment unit (for example,the biological treatment unit 130), the size of the biological treatmentunit may be reduced, in some embodiments by about 30%. There is also alesser requirement for oxygen in the biological treatment unit whichallows for the capacity and power requirements of an aeration system inthe biological treatment unit to also be reduced, in some embodiments byabout 30%. The reduced size of the components of the treatment systemprovides for a decreased footprint of the system. For example, awastewater treatment plant with a capacity to treat 35 million gallonsper day (MGD) of wastewater with an influent BOD of 200 mg/L wouldrequire about 150,000 ft² of treatment units with a conventional designapproach; with embodiments of the present invention the footprint couldbe reduced to about 75,000 ft².

In other embodiments of systems and methods in accordance with thepresent invention, a wastewater treatment system, such as any of thosedescribed above, may further include an anaerobic treatment unit (ananaerobic digester). Non-limiting examples of components or portions ofanaerobic systems that can be utilized in one or more configurations ofthe wastewater treatment systems include, but are not limited to, theDYSTOR® digester gas holder system, the CROWN® disintegration system,the PEARTH® digester gas mixing system, the PFT® spiral guided digestergas holder, the PFT® vertical guided digester holder, the DUO-DECK™floating digester cover, and the PFT® heater and heat exchanger system,from Evoqua Water Technologies.

The anaerobic digester may be utilized to treat mixed liquor, which mayinclude suspended solids, sludge, and/or solids-rich or solids-leanfluid streams, from one or more other treatment units of the wastewatertreatment system. At least a portion of an anaerobically treated sludgeproduced in the anaerobic digester may be recycled back to one or moreother treatment units of the wastewater treatment system. The nature andfunction of the anaerobic digester and associated recycle streams may besimilar to those described in U.S. Pat. No. 8,894,856, titled “Hybridaerobic and anaerobic wastewater and sludge treatment systems andmethods,” which is herein incorporated by reference in its entirety forall purposes.

The systems and components of embodiments of the invention may providecost advantages relative to other wastewater treatment systems throughthe use of biological treatment processes in combination with anaerobicdigestion. The wastewater treatment systems and processes of embodimentsof the present invention can reduce sludge production through the use ofvarious unit operations including aerobic and anaerobic biologicalprocesses and recycle streams. The wastewater treatment processes alsoovercome some of the technical difficulties associated with use of someanaerobic wastewater treatment processes, by, for example, concentratingor strengthening the sludge introduced into the anaerobic digester.Additionally, costs associated with use of a conventional aerobicstabilization unit are typically reduced because less aeration wouldtypically be required in the aerobic processes due to the use of theanaerobic digester and various recycle streams. The various processescan also generate methane as a product of the anaerobic digestionprocess, which can be used as an energy source. In certain embodiments,a large portion of the chemical oxygen demand (COD) and BOD present ininfluent wastewater to be treated can be reduced using the anaerobicdigester. This can reduce the aeration and oxygen requirements, andthus, operation costs of the wastewater treatment system, and increasethe amount of methane produced that can be used as an energy source.Additionally, because anaerobic digestion can be used to reduce COD andBOD in the sludge, the sludge yield can also be reduced. The reductionof COD and/or BOD in the anaerobic treatment unit may also provide for areduction in size of the stabilization tank or other aerobic treatmentunit in the wastewater treatment system as compared to systems notutilizing the anaerobic digester.

Embodiments of the present invention may provide for the recirculationof aerobic bacteria, anaerobic bacteria, or both through various unitoperations of the treatment system.

It was previously believed that methanogens were strict anaerobicbacteria that would die quickly in an aerobic environment. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of methanogenic organisms. One advantageousfeature of the treatment systems of the present application involvesproviding a large amount of methanogens through the anaerobic recycle toa contact stabilization process through the unique internal anaerobicsludge recycle path. At least a portion of the methanogenic bacteriareturn to the anaerobic digester, thereby seeding the anaerobic digesterwith methanogenic bacteria to join the existing population of the viablemethanogens in the anaerobic digester. This reduces the need for theanaerobic digester to have a size and resultant hydraulic residence timeor solids retention time to maintain a stable methanogenic bacteriapopulation in the absence of bacterial seeding, as in previously knownprocesses.

The concentration of seeding methanogenic bacteria, on a basis of acount of microorganisms, provided at the input of the anaerobic digestermay in some embodiments be at least a target percentage, such as about10% or more, of the concentration of the methanogenic bacteria presentin the anaerobically digested sludge stream exiting the anaerobicdigester. In some embodiments, this percentage may be, for example,about 25% or more, about 33% or more, about 50% or more, or about 75% ormore.

The anaerobic digester of systems in accordance with the presentinvention may be sized smaller than those in previously known systems.The methanogenic bacterial seeding of the anaerobic digester alsoprovides for a safety factor against disruptions of the anaerobicdigestion process. In the event of anaerobic digestion process upset orfailure, the anaerobic digesters of the presently disclosed systemswould recover faster than that the anaerobic digesters in previouslyknown systems because the seeding of the anaerobic digester withmethanogenic bacteria would add to the rate of replenishment ofmethanogenic bacteria in the anaerobic reactor due to the growth ofthese bacteria therein, reducing the time required for the anaerobicdigester to achieve a desired concentration of methanogenic bacteria.

The advantage of methanogen recycle can be estimated as follow:

$\theta_{x} = \frac{X_{a}V}{{QX_{a}} - {QX_{a}^{0}}}$

Where

-   -   V=Volume of anaerobic digester    -   θ_(x)=Solids retention time in anaerobic digester (days)    -   X_(a)=concentration of methanogens    -   Q=influent and effluent flow rate    -   X_(a) ⁰=concentration of methanogens in the inlet stream, which        is normally considered zero for conventional activated sludge        process.

If about 50% of methanogens survive in the short solid retention timecontact stabilization process and are recycled back to anaerobicdigester, the solids retention time of the anaerobic digester could bedoubled, or the size of the anaerobic digester decreased by half. Forexample, in previously known systems a hydraulic retention time in ananaerobic digester was in many instances set at between about 20 andabout 30 days. With a treatment system operating in accordance someembodiments of the present application, this hydraulic retention timemay be reduced by about 50% to between about 10 and about 15 days.

In some embodiments of the apparatus and methods disclosed herein, ahydraulic retention time in a treatment system contact stabilizationvessel may be about one hour or less. A significant portion ofmethanogens can be recycled in the short solid retention time contactstabilization aerobic process, which can reduce the capital cost andoperational cost of the anaerobic digester(s). For example, the tankvolume of the anaerobic digester(s) could be decreased to bring thesafety factor to a range closer to those anaerobic digester(s) without amethanogen recycle process. With smaller volume, the capital cost of theanaerobic digesters and the mixing energy consumption of the anaerobicdigestion process would both decrease, which will make apparatus andprocesses in accordance with the present disclosure more cost effectivethan previously known apparatus and processes.

In other embodiments, the seeding of the anaerobic digester withrecycled methanogenic bacteria may provide for decreasing the hydraulicresidence time of sludge treated in the digester. This would result in adecreased cycle time, and thus an increased treatment capacity of thetreatment system. Increasing the amount of methanogens recycled to theanaerobic digester, by, for example, increasing an amount ofmethanogen-containing sludge directed into the digester, would providegreater opportunity to decrease the hydraulic residence time in thedigester and increase the treatment capacity of the system.

If a significant portion of methanogens can be recycled in the aerobiccontact stabilization process, the capital cost and operational cost ofthe anaerobic digesters could be decreased. For example, the tank volumeof the anaerobic digesters could be decreased to bring the safety factorto a range closer to those anaerobic digesters in systems not includinga methanogen recycle process. With smaller volume, the capital cost ofthe anaerobic digesters and the mixing energy consumption of theanaerobic digesters will both decrease, which will make the wastewatertreatment process more cost effective.

In certain embodiments, the contact tank is constantly seeded withnitrification bacteria (such as ammonia oxidizing and nitrite oxidizingbiomass) which can survive the anaerobic digester and which can berecycled back to the aerobic environment. For example, nitrification andde-nitrification can take place in the contact tank. Nitrification maybe carried out by two groups of slow-growing autotrophs:ammonium-oxidizing bacteria (AOB), which convert ammonia to nitrite, andnitrite-oxidizing bacteria (NOB), which oxidize nitrite to nitrate. Bothare slow growers and strict aerobes. In some embodiments of treatmentsystems disclosed herein, the nitrification bacteria are introduced toand/or grown in a contact tank, where they are captured in the floc.Some of the nitrification bacteria will pass out from the contact tankand be sent to an anaerobic digester.

It was previously believed that the strictly anaerobic conditions of theanaerobic digester would kill the nitrification bacteria. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of nitrification organisms in anaerobic andanoxic conditions that may occur in some biological nutrient removalprocesses. Nitrification bacteria which survive the anaerobic digesterand are returned to the aerobic part of the treatment process mayenhance the nitrification process performance in ways that can lowercapital costs, for example by providing for a reduced aerobic treatmentvessel size and/or reduced aerobic treatment hydraulic retention timeand/or an increased safety factor that would render the nitrificationprocess more stable in response to disruptions to the treatment process.Disruptions to the treatment process encompass deviations from desiredoperating parameters which may be caused by, for example, interruptionsin flow of material through the treatment system or a loss oftemperature control at one or more unit operations. The survival rate ofnitrification bacteria in an anaerobic digester could be increased bydecreasing a hydraulic residence time in the anaerobic digester, whichwould be accomplished if the anaerobic digester were seeded withrecycled methanogens, as described above.

A wastewater treatment system, indicated generally at 400 in FIG. 4,includes an anaerobic treatment unit 490, referred to herein as ananaerobic digester. The wastewater treatment system of FIG. 4 includes acontact tank 410, a DAF unit 420, a stabilization tank 430, a clarifier440, and associated fluid conduits 414, 424, 435, 445, 455, 465, and 475which are similar in structure and function to the contact tank 110, DAFunit 120, biological treatment unit 130, clarifier 140, and associatedfluid conduits 114, 124, 135, 145, 155, 165, and 175 of the systemillustrated in FIG. 1 and described above. A singular DAF unit 420 isillustrated in FIG. 4, although in alternate embodiments the treatmentsystem may use multiple DAF units as described above with reference tothe treatment system of FIG. 1.

In the system of FIG. 4, wastewater from a source of wastewater 405 isdirected into a primary clarifier 412 through an inlet of the primaryclarifier. A solids-rich fluid stream from the clarifier is directedthrough conduit 404 into an inlet of a thickener 480, which maycomprise, for example, a gravity belt thickener. A solids-lean effluentfrom the primary clarifier 412 is directed into an inlet of the contacttank 410 through conduit 402. A solids-rich output stream from thethickener 480 is directed to an inlet of the anaerobic digester 490through conduit 484. A solids-lean effluent from the thickener isdirected to an inlet of the contact tank 410 through conduit 482. Theanaerobic digester is also supplied with suspended solids removed frommixed liquor in the DAF unit 420 through conduits 425 and 484.

In some embodiments, a portion of the suspended solids removed from themixed liquor in the DAF unit 420 may be recycled to the contact tank 410through conduits 425 and 426 in a similar manner as the recycle ofsuspended solids removed in the DAF unit(s) 120 to the contact tank 110described above with reference to FIG. 1.

The solids-rich output stream from the thickener 480 and any suspendedsolids from the DAF unit 420 introduced into the anaerobic digester 490are combined and anaerobically digested in the anaerobic digester. Theanaerobic digestion process can be operated at temperatures betweenabout 20° C. and about 75° C., depending on the types of bacteriautilized during digestion. For example, use of mesophilic bacteriatypically requires operating temperatures of between about 20° C. andabout 45° C., while thermophilic bacteria typically require operatingtemperatures of between about 50° C. and about 75° C. In certainembodiments, the operating temperature may be between about 25° C. andabout 35° C. to promote mesophilic activity rather than thermophilicactivity. Depending on the other operating parameters, the retentiontime in the anaerobic digester can be between about seven and about 50days retention time, and in some embodiments, between about 15 and about30 days retention time. In certain embodiments, anaerobic digestion ofmixed liquor in the anaerobic digester may result in a reduction inoxygen demand of the mixed liquor of about 50%.

A first portion of an anaerobically digested sludge produced in theanaerobic digester may be recycled through an outlet of the anaerobicdigester and into the stabilization tank 430 through conduit 492. Thisrecycle stream may facilitate retaining sufficient solids in the systemto provide a desired residence time in the stabilization tank. Theanaerobically digested sludge recycled to the stabilization tank mayalso seed the stabilization tank with nitrification bacteria to enhancethe nitrification activity within the stabilization tank as describedabove. The anaerobically digested sludge recycled into the stabilizationtank may also contain methanogenic bacteria which are subsequentlyreturned to the anaerobic digester to enhance the performance of theanaerobic digester as described above.

In embodiments where the stabilization tank 430 includes an aeratedanoxic region and an aerobic region, such as in the biological treatmentunit 130 of FIG. 2 described above, the portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank. A second portion ofthe anaerobically digested sludge produced in the anaerobic digester maybe sent out of the system as waste solids through a conduit 495. Thefirst portion of the anaerobically digested sludge recycled into thestabilization tank 430 may be any amount between about 0% and about 100%of the anaerobically digested sludge produced in and output from theanaerobic digester, with the second portion, making up the balance, sentout of the system as waste solids through conduit 495. In someembodiments, between about 0% and about 80% of the anaerobicallydigested sludge is recycled from one or more outlets of the anaerobicdigester to one or more other unit operations of the treatment system.

In another embodiment of the wastewater treatment system, indicatedgenerally at 500 in FIG. 5, the first portion of the anaerobicallydigested sludge produced in the anaerobic digester is recycled throughan outlet of the anaerobic digester and into the inlet of the contacttank 410 through conduit 494, rather than into the stabilization tank430. This recycle stream may facilitate providing sufficient activatedsludge in the contact tank to adsorb/absorb or enmesh BOD present in theinfluent wastewater. The anaerobically digested sludge recycled to thecontact tank may also seed the contact tank with nitrification bacteriato enhance the nitrification activity within the contact tank asdescribed above. The anaerobically digested sludge recycled into thecontact tank may also contain methanogenic bacteria which aresubsequently returned to the anaerobic digester to enhance theperformance of the anaerobic digester as described above. The firstportion of the anaerobically digested sludge recycled into the contacttank 410 may be any amount between about 0% and about 100% of theanaerobically digested sludge produced in and output from the anaerobicdigester, with a second portion, making up the balance, sent out of thesystem as waste solids through conduit 495.

In another embodiment of the wastewater treatment system, indicatedgenerally at 600 in FIG. 6, a first portion of the anaerobicallydigested sludge produced in the anaerobic digester may be recycledthrough an outlet of the anaerobic digester and into the inlet of thecontact tank 410 through conduit 494, and a second portion of theanaerobically digested sludge may be recycled through an outlet of theanaerobic digester and into the stabilization tank 430 through conduit492. These recycle streams may provide the benefits described above withregard to systems 400 and 500. A third portion of the anaerobicallydigested sludge may be directed to waste through conduit 495. The sum ofthe first portion of the anaerobically digested sludge and the secondportion of the anaerobic sludge may be any amount between about 0% andabout 100% of the anaerobically digested sludge produced in and outputfrom the anaerobic digester, with the third portion, making up thebalance, sent out of the system as waste solids through conduit 495. Therecycled anaerobic sludge may be split in any desired ratio between thefirst portion and the second portion. The first potion may comprise fromabout 0% to about 100% of all the anaerobically digested sludge producedin and output from the anaerobic digester with the sum of the secondportion and the third portion making up the balance.

Another embodiment of the wastewater treatment system, indicatedgenerally at 700 in FIG. 7, is similar to that illustrated in FIG. 6,however the thickener 480 is not utilized. Rather, the solids-rich fluidstream from the clarifier is directed through conduit 406 into an inletof the DAF unit 420. The DAF unit 420 of the system illustrated in FIG.7 performs the function of the thickener 480 of the system illustratedin FIG. 6. The utilization of the DAF unit 420 to perform the functionof the thickener may reduce or eliminate the need for a thickener in thesystem, which may reduce both capital and operational costs of thesystem. A first portion of the anaerobically digested sludge created inthe anaerobic digester 490 is recycled to the contact tank 410 and asecond portion is recycled to the stabilization tank 430 to provide thebenefits described above. A third portion of the anaerobically digestedsludge is directed to waste through conduit 495.

Further embodiments may include any combination of features of thesystems described above. For example, in some embodiments, a firstportion of the solids-rich fluid stream from the clarifier is directedthrough conduit 406 into an inlet of the DAF unit 420, while a secondportion is directed into a thickener 480. In any of the aboveembodiments, the stabilization tank 430 may include an aerated anoxicregion and an aerobic region. A first portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank and a second portionmay be recycled to the aerobic region. The ratio the amount of recycledanaerobic sludge directed to the aerated anoxic region to the amount ofrecycled anaerobic sludge directed to the aerobic region may be anyratio desired. Any of the embodiments disclosed herein may includemultiples of any of the treatment units and/or conduits illustrated.

In accordance with another embodiment, methods disclosed herein maycomprise fermenting floated biosolids to produce soluble chemical oxygendemand (sCOD). The fermentation may generally occur in an anaerobicenvironment. Downstream, the fermented solids may be directed to abiological treatment unit to promote biological breakdown of the DAFeffluent.

Harmful nutrient compounds, for example, nitrogen and phosphorous, inmunicipal wastewater treatment plant discharge may cause culturaleutrophication (nutrient enrichment due to human activities) in surfacewaters. Summer algal blooms are an example of cultural eutrophicationwhich causes low dissolved oxygen, fish kills, murky waters, anddepletion of desirable flora and fauna. Biological nutrient removal inwastewater treatment system can be employed to remove nitrogen andphosphorous from wastewater to remediate cultural eutrophication.

Fermentation of the floated biosolids may promote enhanced biologicalphosphorus removal by the production of soluble organic carbon, forexample, volatile fatty acids (VFAs). Biological reactors may havephosphate accumulating organisms (PAOs) which consume soluble carboncompounds, such as VFAs, and incorporate phosphorous into cell biomass,which can be removed as waste sludge. Briefly, during the anaerobicphase of biological treatment, VFAs may be consumed by PAOs andconverted to polyhydroxy alkanoates (PHA). The mechanism requires energywhich may be obtained by the bacteria discharging ortho-phosphate,complexed as polyphosphate inside the cell. During aerobic conditions,the bacteria may oxidize PHA and consume ortho-phosphate to replenishpolyphosphate stores. The mechanism may result in a net uptake ofphosphorous due to the growth of the bacteria. Downstream, the excessbiomass may be removed from the system as waste sludge. A similarmechanism for nitrogen removal may occur during the anoxic phase.

Often, a soluble organic carbon concentration in municipal wastewater istoo low to achieve adequate phosphorous removal. Certain municipalwastewater has a raw sewage VFA concentration between about 5 and 15mg/L as acetic acid (HAc). Conventionally, process streams aresupplemented with VFAs or other sCOD to drive phosphorous removal. Forinstance, process streams may be supplemented to have a concentration ofat least about 15 mg/L VFA as HAc. The methods disclosed herein may beused to treat wastewater having a low soluble organic carbonconcentration, which may otherwise require supplementation with carboncompounds or VFAs for adequate phosphorous removal. The targetconcentration of soluble organic carbon to be supplied to the biologicaltreatment unit may generally depend on the quality of the influentwastewater.

The methods disclosed herein may produce soluble organic carbon withinthe system by solids fermentation, alleviating or removing the need tosupplement wastewater with carbon compounds for treatment. In someembodiments, fermentation of the floated solids may increase VFAconcentration in the process stream to be at least about 17 mg/L VFA asHAc, for example, at least about 20 mg/L VFA, at least about 24 mg/LVFA, at least about 30 mg/L VFA, at least about 50 mg/L VFA, and in somecases up to about 60 mg/L VFA as HAc. The concentration of VFA in thefermented solids may generally depend on the quality of the incomingwastewater and/or the design and operation parameters of thefermentation unit. The fermented solids may biologically treat the DAFeffluent by reducing a concentration of dissolved phosphorous whenforming the biologically treated mixed liquor. In accordance withcertain embodiments disclosed herein, an influent concentration ofphosphorous of between about 5 mg/L to about 7.5 mg/L may be reduced toless than 0.15 mg/L, for example, less than about 0.1 mg/L phosphorousin the biologically treated effluent.

The systems and methods disclosed herein may be operated by controllingan amount of floated solids directed to be fermented and/or controllingan amount of fermented solids directed to the biological treatment unit.In some embodiments, methods may comprise determining a target sCODdemand of the biological treatment unit. The determination may be madeby measuring the composition of the process water upstream from thebiological treatment unit, for example, at the inlet or in an upstreamvessel. In particular, soluble organic carbon or VFA concentration inthe process water may be measured. The determination may be made bymeasuring composition of the process water in the biological treatmentunit or downstream of the biological treatment unit, for example, at anoutlet of the biological treatment unit or of the system. In particular,phosphorous and/or dissolved phosphorous concentration in the processwater or system effluent may be measured. In some embodiments,composition of the activated sludge may be measured. Additionally oralternatively, the determination may be made by calculating a targetsCOD demand of the biological treatment unit, for example, by reviewinghistorical sCOD demand data.

The method may further comprise selectively directing floated solids tobe fermented and/or selectively directing fermented solids to thebiological treatment unit responsive to the sCOD demand and in an amountsufficient to meet the sCOD demand. In some embodiments, an amountsufficient to meet the sCOD demand includes an amount sufficient torestore sCOD demand to be within tolerance of a target sCOD demand ofthe biological treatment unit. The target sCOD demand of the biologicaltreatment unit is an amount associated with a target phosphorous ordissolved phosphorous concentration in the system effluent. In someembodiments, the effluent discharge limit may be <0.1 mg/l of totalphosphorus. In some embodiments the target sCOD demand in an amountassociated with a target phosphorus or dissolved phosphorusconcentration in the system effluent would be about 8 mg sCOD/mg P,where P=mg P removed.

The amount of floated solids directed to a fermentation operation mayrange from about 1% to about 100% of a total amount of floated solidsremoved from the DAF unit(s). The amount of floated solids directed to afermentation operation may be a majority of a total amount of floatedsolids removed, for example, greater than about 50%, between about 50%and about 95%, or between about 60% and about 80% of the total amount offloated solids removed. In some embodiments, the amount of floatedsolids directed to a fermentation unit is greater than the amount offloated solids directed to an anaerobic digester. In other embodiments,the amount of floated solids directed to a fermentation operation may bea minority of a total amount of floated solids removed, for example,less than about 50%, between about 50% and about 5%, or between about40% and about 10% of the total amount of floated solids removed. In someembodiments, the amount of floated solids directed to a fermentationunit is less than the amount of floated solids directed to an anaerobicdigester.

The amount of fermented solids directed to the biological treatment unitmay range from about 1% to about 100% of a total amount of fermentedsolids removed from a fermentation operation. The amount of fermentedsolids directed to a biological treatment unit may be a majority of atotal amount of fermented solids removed, for example, greater thanabout 50%, between about 50% and about 95%, or between about 60% andabout 80% of the total amount of fermented solids removed. In someembodiments, the amount of fermented solids directed to a biologicaltreatment unit is greater than the amount of fermented solids directedto any other unit operation, for example, a contact tank or an anaerobicdigester. In other embodiments, the amount of fermented solids directedto a biological treatment unit may be a minority of a total amount offermented solids removed, for example, less than about 50%, betweenabout 50% and about 5%, or between about 40% and about 10% of the totalamount of fermented solids removed. In some embodiments, the amount offermented solids directed to a biological treatment unit is less thanthe amount of fermented solids directed to any other unit operation, forexample, a contact tank or an anaerobic digester.

Fermentation may occur in any anaerobic environment. In general,fermentation may occur in the presence of anaerobic bacteria. A pH of 4or greater, for example, of 5.5 or greater, may be maintained during thefermentation operation to prevent inhibition of VFA production. In someembodiments, a fermentation unit may be positioned between the DAF unitand the biological treatment unit to promote fermentation of the floatedsolids. Methods of retrofitting a treatment system may include providinga fermentation unit and fluidly connecting the fermentation unit asshown in the figures described below. The fermentation unit may be anon-thickening fermenter, for example, a complete mix fermenter, or athickening fermenter, for example, a single stage, two stage, or unifiedstage fermenter/thickener. The fermentation unit may comprise a staticfermenter or a complete mix fermenter. The fermentation unit may be aseparate complete mix thickener fermenter. In general, the supernatantof the fermentation unit may be recycled (for example, back to thecontact tank) and the fermented sludge may be directed to the biologicaltreatment unit and/or anaerobic digester. For a fermenter and thickenerunit, the thickened sludge may be directed to the biological treatmentunit and/or anaerobic digester or recycled back to the fermenter.

In other embodiments, a system without a fermentation unit may bearranged or operated to promote fermentation of floated biosolids in anexisting vessel before transferring the fermented solids to thebiological treatment unit. For instance, system 700 may be arranged topromote fermentation of floated solids into sCOD in anaerobic digester490. Downstream, fermented solids may be directed to biologicaltreatment unit 430 through conduit 492 as needed. A balance of thefermented solids may be removed through conduit 495. Alternatively,system 1000 as arranged may be retrofitted by fluidly connecting anoutlet of the anaerobic digester 490 to an inlet of the biologicaltreatment unit 430 (for example, via a conduit similar to 492 as shownin FIG. 4), such that floated biosolids fermented in the anaerobicdigester 490 may be directed to the biological treatment unit 430, aspreviously described.

Methods may further comprise providing and/or installing controllers,valves, pumps, fluid conduits, or vessels to treat the wastewater asdescribed. The methods may further include instructing, advising, orotherwise informing a user to operate the system to provide solubleorganic carbon to the biological treatment unit, as described herein.

The solids residence time (SRT) of the mixed liquor in the fermentationunit is in some embodiments from about 2 days to about 15, and in someembodiments, from about 3 days to about 6 days. The hydraulic residencetime (HRT) of the mixed liquor in the fermentation unit is generallyless than the SRT. In some embodiments, the HRT is from about 4 hours toabout 28 hours, and in some embodiments, from about 6 hours to about 12hours. The residence time may be adjusted depending upon factors such asthe temperature of the reactor and COD of the influent floatedbiosolids. For instance, the SRT may be from about 2 days to about 3days for temperatures greater than 20° C. The HRT for such temperaturemay be from about 4 hours to about 8 hours. In general, the residencetime may be sufficient to produce sCOD at a rate required by thebiological treatment unit. The sCOD demand of the biological treatmentunit may be dependent on factors such as the quality of the influentwastewater and upstream treatment.

Another embodiment, indicated generally at 1300, is provided in FIG. 13.The system 1300 includes a contact tank 610, a DAF unit 620, astabilization tank 630 (biological treatment unit), a fermentation unit670, and associated fluid conduits 614, 624, and 635, which are similarin structure and function to elements of the systems illustrated inFIG. 1. The system 1300 includes fluid conduit 623 extending between theDAF unit 620 and the fermentation unit 670. The system 1300 includesfluid conduit 674 extending between the fermentation unit 670 and thebiological treatment unit 630. The system 1300 includes screen 613positioned in the fluid conduit 623 to reduce an amount ofnon-biological waste solids entering the fermentation unit 670. Two DAFunits 620 are illustrated in FIG. 13, although in alternate embodimentsthe treatment system may use a single DAF unit as shown, for example, inthe treatment system of FIG. 4.

In system 1300, wastewater from a source of wastewater 605 is directedinto a contact tank 610 through an inlet of the contact tank. Thewastewater may be mixed with activated sludge to form an activated mixedliquor. Downstream from the contact tank 610, gas from a source of gas608 is directed into DAF unit 620 to float suspended solids. Furtherdownstream, the process liquid flows through stabilization tank 630, aspreviously described herein.

Methods disclosed herein may include separating non-biological wastesolids from the floated solids. Non-biological waste solids may includetrash and other non-degradable solids that are not desired in thesystem. In system 1300, floated solids from the DAF unit 620 aredirected to a fermentation unit 670 through conduit 623. Non-biologicalwaste solids are retained by screen 613. The fermentation unit 670 maybe configured to ferment solids in an anaerobic environment beforedirecting the solids to the biological treatment unit 630 via conduit674.

Another embodiment indicated generally at 1400 is shown in FIG. 14.System 1400 is similar to system 1300 but further includes a clarifier640 (solids-liquid separation unit) and associated fluid conduits 645,655, 665, and 675, which are similar in structure and function toelements of the system illustrated in FIG. 1. System 1400 furtherincludes fluid conduit 671 extending between the fermentation unit 670and contact tank 610. The system 1400 includes screen 611 positioned inthe fluid conduit 671 to reduce an amount of non-biological waste solidsentering the contact tank 610.

Another embodiment indicated generally at 1500 is shown in FIG. 15.System 1500 is similar to system 1400 but further includes an anaerobicdigester 690 and associated fluid conduit 695, which are similar instructure and function to anaerobic digester 490 and fluid conduit 495illustrated in FIG. 4. System 1500 includes fluid conduit 628 extendingbetween the DAF unit 620 and the anaerobic digester 690. The system 1500includes screen 618 positioned in the fluid conduit 628 to reduce anamount of non-biological waste solids entering the anaerobic digester690. System 1500 includes two screens 613 and 618, but in certainembodiments, the system may include a single screen upstream from asingle fluid conduit which splits into conduits 623 and 628. The systemmay further include a metering valve positioned upstream from fluidconduits 623 and 628 to selectively direct floated biosolids to thefermentation unit 670 and anaerobic digester 690.

System 1500 includes controller 638 and metering valve 677 which areoperably connected to each other. Metering valve 677 may be positionedand configured to selectively direct fermented solids from fermentationunit 670 to biological treatment unit 630 (through conduits 674 and 676)and anaerobic digester 690 (through conduits 674 and 678). Controller638 may be configured to instruct the metering valve 677 to selectivelydirect fermented solids, as previously described. Metering valve 677 isconfigured to selectively direct flow through fluid conduits 676 and678, but in certain embodiments, a metering valve may be positioned toselectively direct flow through fluid conduits 676, 678, and 671.

Another embodiment indicated generally at 1600, is illustrated in FIG.16. In this embodiment, the biological treatment unit 630 includes ananaerobic region 652, an aerated anoxic region 660, an aerobic region650, and a post-anoxic region 662. The aerated anoxic region 660 is influid communication downstream of the anaerobic region 652 and receivesbiologically treated anaerobic mixed liquor from the anaerobic region652. The aerobic region 650 is in fluid communication downstream of theaerated anoxic region 660 and receives biologically treated anoxic mixedliquor from the aerated anoxic region 660. The post-anoxic region 662 isin fluid communication downstream of the aerobic region 650 and receivesbiologically treated aerobic mixed liquor from the aerobic region 650.

In some embodiments, one or more of the anaerobic region 652, theaerated anoxic region 660, the aerobic region 650, and the post-anoxicregion 662 may be formed in a same vessel or tank and separatedtherefrom by a partition or weir. In other embodiments, one or more ofthe anaerobic region 652, the aerated anoxic region 660, the aerobicregion 650, and the post-anoxic region 662 may be physically separatefrom one or more other regions. For example, the aerobic region 650 andthe aerated anoxic region 660 may occupy distinct vessels or tanks ormay be otherwise separated from one another. The contact tank 610 andthe aerated anoxic region 660 may be associated by fluid conduit 615, asshown in FIG. 16, or may be formed in a same vessel or tank.

In system 1600, fluid conduit 624 which extends between the DAF unit 620and biological treatment unit 630 is split into fluid conduits 627 a and627 b directed to the anaerobic region 652 and aerobic region 650,respectively. The wastewater treatment system may contain one or both ofthese conduits. In embodiments in which the wastewater treatment systemcontains both conduits 627 a and 627 b, the system may further include ametering valve positioned upstream from conduits 627 a and 627 bconfigured to selectively direct floated solids to the anaerobic region652 and the aerobic region 650 (similar to metering valve 679 associatedwith fluid conduits 676 a, 676 b, and 676 c, shown in FIG. 17). Themetering valve may optionally be operably connected to a controller,such as controller 638 shown in FIG. 17. In other embodiments, theeffluent from the DAF unit(s) 620 may be introduced into the aeratedanoxic region 660 and then directed into the aerobic region 650.

In system 1600, fluid conduit 674 which extends between the fermentationunit 670 and the biological treatment unit 630 is split into fluidconduits 676 a, 676 b, and 676 c directed to the anaerobic region 652,aerated anoxic region 660, and post-anoxic region 662, respectively. Thewastewater treatment system may contain one or more of these conduits.

Another embodiment indicated generally at 1700 is shown in FIG. 17.System 1700 is similar to system 1600 but further includes controller638 operably connected to metering valve 679. Metering valve 679 may bepositioned and configured to selectively direct fermented solids fromfermentation unit 670 to anaerobic region 652 (through conduits 674 and676 a), aerated anoxic region 660 (through conduits 674 and 676 b),and/or post-anoxic region 662 (through conduits 674 and 676 c).Controller 638 may be configured to instruct the metering valve 679 toselectively direct fermented solids, as previously described.

Controller 638 is operably connected to metering valve 677 (as shown inFIG. 15) and metering valve 679 (as shown in FIG. 17), but in certainembodiments separate controllers may be provided for each meteringvalve. Methods disclosed herein may comprise programming the controllerto operate the system, as described. For instance, the controller 638may be programmed to operate metering valve 677 and/or 679automatically, for example, on a schedule or responsive to a measurementor calculation received or determined by the controller 638. Forexample, in some embodiments, the controller 638 may obtain ameasurement associated with the composition of the mixed liquor in thebiological treatment unit 630. The controller 638 may operate one ormore metering valve (677 and/or 679) to adjust treatment of the mixedliquor in the biological treatment unit. The controller 638 may operateone or more pumps to adjust treatment of the wastewater in the system.The measurement may be input manually or obtained from a sensor operablyconnected to the controller 638. Any controllers, sensors, meteringvalves, or pumps known to one of ordinary skill in the art may beprovided to operate as described herein.

EXAMPLES Example 1

A wastewater treatment system 1000 was configured as illustrated in FIG.10, where the indicated unit operations and conduits have the samestructure and function as the identically indicated unit operations andconduits in FIGS. 4-7. The wastewater treatment system 1000 was used toexamine the effects of recycling removed solids from the DAF unit 420 tothe contact tank 410. By gradually increasing the amount of removedsolids from the DAF unit 420 recycled to the contact tank 410 from 0% ofthe solids removed in the DAF unit to about 90% of the solids removed inthe DAF unit over the course of three weeks, the suspended solids (MLSS)content of contact tank was brought up from 600 mg/L to over 1200 mg/L.The DAF dissolved solids content increased from 3%-4% prior to beginningthe recycle of solids from the DAF unit to the contact tank to above 5%after beginning the recycle of solids from the DAF unit to the contacttank. The total suspended solids (TSS) removal efficiency of the DAFunit increased from about 75% to over 85%. The COD removal of the DAFunit increased from about 70% to about 80% over the course of thetesting. These results are illustrated in the charts of FIG. 11 and FIG.12.

These results show that recycling removed solids from a DAF unit to acontact tank in a system such as that illustrated in FIG. 10 may providefor a greater amount of suspended solids in the contact tank. Theincreased amount of suspended solids in the contact tank increases theamount of suspended and soluble COD and BOD which may be removed fromwastewater influent to the contact tank and absorbed/adsorbed/enmeshedin the suspended solids and/or which may be oxidized in the contacttank. Recycling removed solids from a DAF unit to a contact tank in asystem such as that illustrated in FIG. 10 increases the efficiency ofthe removal of suspended solids in the DAF unit. These effects maydecrease the load on downstream unit operations and may reduce operatingcosts of the system as a whole and/or may reduce capital costs of thesystem by providing for smaller downstream processing units to beutilized. Further, a greater amount soluble BOD/COD from wastewaterinfluent to the system may be removed as solids in the DAF unit and maybe sent from the DAF unit to an anaerobic digester instead of an aerobictreatment unit operation, reducing the aeration power requirements ofthe system and increasing the amount of biogas that could be produced.

Prophetic Example 1

In this prophetic example, a water treatment system was configured asillustrated in FIG. 1 with the biological treatment unit 130 comprisinga single tank.

Assumptions of Feed:

The system was fed wastewater at a rate of 57,600 gallons/day (gpd), 40gallons per minute (gpm). The wastewater was assumed to be typical ofmunicipal wastewater, having a total BOD (tBOD) of 140 mg/l (67 lbs/day)of which 43% (60 mg/l, 29 lbs/day) was particulate (non-soluble) BOD(pBOD), and 57% (80 mg/l, 38 lbs/day) was soluble BOD (sBOD). Thewastewater was also assumed to include 100 mg/l (48 lbs/day) ofsuspended solids (SS), of which 19 lbs/day (48 lbs/day SS−29 lbs/daypBOD) was assumed to be inert (non-biological) material, and 6 lbs/dayof ammonia.

HDT Assumptions:

The hydraulic detention time (HDT) in the contact tank 110 was assumedto be 45 minutes and the hydraulic detention time (HDT) in thebiological treatment unit 130 was assumed to be five hours.

Flow Rate Through Contact Tank:

The ratio of return sludge sent from the clarifier 140 to the contacttank was set at 2.4 lb/lb of tBOD, for a (2.4)(67 lbs/day tBOD)=160lbs/day recycled sludge or 2,880 gpd (2.0 gpm), assuming a recycledsludge solids loading of 6,660 mg/l. The total flow through the contacttank was thus 57,600 gpd+2,880 gpd=60,480 gpd (42 gpm).

From laboratory bench scale testing, it was found that in the contacttank, approximately 50% of the sBOD was removed, with approximately ⅔ ofthe amount removed converted to SS, and approximately ⅓ of the amountremoved oxidized, for example, converted to carbon dioxide and water.Thus, it was assumed that in the contact tank 14 lbs/day of sBOD wasconverted to SS and 5 lbs/day of pBOD was oxidized. The total solidspassed through the contact tank was thus 160 lbs/day recycled sludge+48lbs/day suspended solids from influent wastewater+14 lbs/day sBODconverted to SS−5 lbs pBOD oxidized=217 lbs/day. The mixed liquorsuspended solids (MLSS) leaving the contact tank was thus ((217lbs/day)/(60,480 gpd))(453592.4 mg/lb)(0.2641721 gal/l)=430 mg/l.

The tBOD leaving the contact tank was 67 lbs/day input−5 lbs/dayoxidized=62 lbs/day (121 mg/l). The sBOD leaving the contact tank was 38lbs/day in−14 lbs/day converted to SS−5 lbs/day oxidized=19 lbs/day (37mg/l). The pBOD leaving the contact tank was 29 lbs/day influent+14lbs/day converted from sBOD=43 lbs/day (84 mg/l).

Flow Split into DAF and Biological Treatment Tank:

The flow out of the contact tank was split between the DAF units 120 andthe biological treatment unit 130. 46.5% (101 lbs/day, 28,080 gpd, 19.5gpm) of the output of the contact tank was directed to the DAF units and53.5% (116 lbs/day, 32,400 gpd, 22.5 gpm) was directed into thebiological treatment unit.

It was assumed that all recycled sludge directed to the DAF units (160lbs/day introduced into contact tank−116 lbs/day returned to biologicaltreatment tank=44 lbs/day) was removed in the DAF process.

BOD Influent to Biological Treatment Unit:

The total BOD to be treated in the biological treatment unit includesthe BOD from the contact tank (53.5% of 62 lbs/day=33 lbs/day) in 32,400gpd of influent plus BOD from the DAF units. The pBOD influent to theDAF units was 46.5% of 43 lbs/day output from contact tank=20 lbs/day.The sBOD influent to the DAF units was 46.5% of 19 lbs/day output fromthe contact tank=9 lbs/day at a flow rate of 28,800 gpd. Assuming 80% ofthe pBOD was removed in the DAF units, the tBOD flowing from the DAFunits to the biological treatment tank was (0.2*20 lbs/day pBOD)+9lbs/day sBOD=13 lbs/day tBOD. Thus the total influent BOD to thebiological treatment tank was 33 lbs/day from the contact tank+13lbs/day from the DAF units=46 lbs/day.

Solids in Biological Treatment Tank:

The biological treatment unit was sized to accommodate a BOD loading of29 lbs/1000 ft³, a common loading in the industry. This meant that thevolume of the biological treatment unit was (46 lbs/day influenttBOD)/(29 lbs/1000 ft³ tBOD loading)=1,600 ft³ (12,000 gal). This volumeresulted in a HDT in the biological treatment unit of (12,000 gal/57,600gpd)(2 hr/day)=5 hours. The total solids in the biological treatmentunit was set at 220 lbs, for a total MLSS of (220 lbs/12,000 gal)(0.264gal/l)(453,592 mg/lb)=2200 mg/l. Assuming a sludge yield of 95% of theBOD results in an amount of waste sludge produced in the biologicaltreatment unit of (0.95)(46 lbs/day tBOD)=44 lbs/day waste sludge. Thewaste sludge age would thus be (220 lbs total solids)/(44 lbs/day wastesludge)=5.2 days.

Biological Treatment Tank Oxygen Requirements:

It was assumed that 0.98 lbs of oxygen were required to oxidize a poundof BOD and 4.6 lbs of oxygen were required to oxidize a pound ofammonia. The oxygen requirement of the biological treatment unit wasthus (0.98 lbs O₂/lb BOD)(46 lbs tBOD/day)+(4.6 lbs 02/lb ammonia)(6lb/day ammonia)=72.6 lb/day O₂ (3 lb O₂/hr). Using a FCF (FieldCorrection Factor—a correction factor to compensate for the reducedoxygen absorbing ability of mixed sludge in the biological treatmenttank as opposed to clean water) of 0.5, this results in a specificoxygen utilization rate (SOUR) of 6 lbs O₂/hr. Assuming diffused air wassupplied to the biological treatment tank from a aeration systemsubmerged by nine feet and a 6% oxygen transfer capability (OTE), thebiological treatment unit would require a flow of (6 lbsO₂/hr)(1/0.06)(1/60 hour/min)(1/1.4291/g O₂)(453.6 g/lb)(0.035ft³/l)=18.5 ft³/min (scfm), or if aerating with air with approximately20% O₂, 92.6 scfm.

Clarifier:

The clarifier was assumed to have a 61 ft³ volume. 57,600 gpd flowedinto the clarifier, resulting in an overflow of 57,600 gpd/61 ft³=944gallon per ft³ per day (gpcfd) overflow rate. Assuming an MLSS of 2200mg/l from the biological treatment tank and targeting a recycled sludge(RAS) concentration of 6600 mg/l and 50% of overflow recycled as RASgives a RAS flow rate of 20 gpm (28,800 gpd). It was assumed that 18 gpmRAS was recycled to the biological treatment tank and 2 gpm to thecontact tank. The solids loading of the clarifier was thus (57,600 gpdinfluent wastewater+28,800 gpd RAS)(2200 mg/l MLSS)(1/453592.4lb/mg)(3.79 l/gal)/(61 ft³)=(1588 lbs/day)/ (61 ft³)=26 lb/ ft³day.

Solids Wasted:

Solids wasted in DAF units: 101 lbs/day (assuming 100% efficiency).

Ratio of sludge wasted to BOD treated: (101 lbs/day)/(67 lbs/day tBOD inwastewater influent)=1.5

With the addition of the DAF units to the treatment system in the aboveexample, the amount of tBOD to be treated in the biological treatmenttank was reduced from 62 lbs/day to 46 lbs/day, a reduction of 26%. Thisprovided for a reduced required size for the biological treatment tankto obtain a desired solids loading and resulted in a decrease in therequired amount of air needed to treat this tBOD in the biologicaltreatment tank. This would translate into a cost savings for bothcapital costs, for a reduced size of the biological treatment tank andaeration system, as well as a decreased operating cost due to thereduced amount of aeration required.

Prophetic Example 2

A simulation was performed using BIOWIN™ simulation software (EnviroSimAssociates Ltd., Ontario, Canada) to compare the performance of awastewater treatment system in accordance with an embodiment of thepresent invention with and without an anaerobic sludge recycle.

The wastewater treatment system without the anaerobic sludge recycleincluded was configured as illustrated in FIG. 8, indicated generally at800. This system is similar to that illustrated in FIG. 4, but with noanaerobic sludge recycle conduit 492 and with the addition of a membranebioreactor (MBR) 510 which receives a solids lean effluent from theclarifier 440 through conduit 442. The MBR produces a product waterpermeate which is removed from the system through conduit 445, and asolids-rich retentate, which is recycled to the DAF unit 480 throughconduit 444. The MBR 510 was simulated to perform complete nitrificationof the solids lean effluent from the clarifier 440.

The performance of the wastewater treatment of FIG. 8 was simulated andcompared to the simulated performance of the wastewater treatment system900 of FIG. 9. Wastewater treatment system 900 of FIG. 9 is similar towastewater treatment system 800 of FIG. 8, but with the addition of ananaerobic sludge recycle conduit 492 recycling anaerobically digestedsludge from the anaerobic digester 490 to the stabilization tank 430though conduit 492. In the simulation of the wastewater treatment system900, 45% of the anaerobically digested sludge output from the anaerobicdigester 490 was recycled to the stabilization tank 430, and 55% of theanaerobically digested sludge output from the anaerobic digester 490 wassent to waste.

The simulation of the performance of both systems 800 and 900 assumed aninfluent wastewater flow rate of 100 MGD. The influent wastewater wasassumed to have a COD of 500 mg/L, a total suspended solids (TSS) of 240mg/L, a Total Kjeldahl Nitrogen (TKN) of 40 mg/L, and a temperature of15° C.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in a decrease in the totaloxygen requirement for treating the influent wastewater as compared tothe system 800 of from 113,891 kg O₂/day to 102,724 kg O₂/day, a savingsof about 10%. Assuming an oxygen transfer energy requirement of 1.5 kgO₂/kwh, this reduction in oxygen consumption would reduce the powerrequirements associated with providing the oxygen from 75,988 kwh/day to68,483 kwh/day, a savings of 7,515 kwh/day.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in an increase in the amountof methane produced as compared to the system 800 from 1,348 scfm to1,652 scfm, an increase of about 23%. Assuming that 35% of the methanechemical energy could be converted to electricity, the potentialelectricity generation from the methane produced would increase from104,511 kwh/day to 128,989 kwh/day.

Combining the energy reduction from the reduced oxygen requirement withthe energy gain from the increased methane production results in anenergy savings of about 31,982 kwh/day for the system 900 including theanaerobically digested sludge recycle as compared to the system 800without the anaerobically digested sludge recycle.

The results of the simulation also indicated that adding theanaerobically digested sludge recycle of the system 900 to the system800 resulted in a reduction in biomass (sludge) production from 81,003pounds per day to 61,167 pounds per day, a reduction of about 25%.

This simulation data indicates that the addition of an anaerobicallydigested sludge recycle to wastewater treatment systems in accordancewith the present invention may result in a significant reduction inpower consumption and a significant decrease in waste sludge production,both of which would result in cost savings and enhancedenvironmental-friendliness of the wastewater treatment system.

Prophetic Example 3

Calculations were performed to compare the performance of a wastewatertreatment system in accordance with an embodiment of the presentinvention with and without a recycle of solids removed in a DAF unit ofthe system to a contact tank of the system. The wastewater treatmentsystem was configured as illustrated in FIG. 10.

It was assumed that the system was provided with 40 million gallons perday of wastewater influent with a BOD level of 250 mg/L (83,400 lbs/day)and suspended solids of 252 mg/L (84,000 lbs/day).

It was assumed that the biological treatment tank 430 operated with asolids retention time (SRT) of 5 days, a mixed liquor suspended solids(MLSS) concentration of 3,000 mg/L and a BOD loading of 45 lbs/1,000cubic feet (20.4 kg/28.3 cubic meters) and that all solids separated inthe clarifier 440 were recycled to the contact tank 410. The hydraulicdetention time (HDT) of the contact tank 410 was assumed to be 25minutes for the system operating without the DAF to contact tank solidsrecycle and one hour for the system operating with the DAF to contacttank solids recycle. The increase in HDT in the contact tank for thesystem when operating with the DAF to contact tank solids recycle was toprovide for the increased MLSS in the contact tank to adsorb additionalsoluble BOD in the contact tank as compared to the system operatingwithout the DAF to contact tank solids recycle. For the system operatingwith a recycle of solids from the DAF unit to the contact tank, it wasassumed that the DAF unit removed 308,000 lbs/day (139,706 kg/day) ofsolids from the mixed liquor passing through it and recycled 190,000lbs/day (86,183 kg/day, 62% of the solids removed) to the contact tankwhile directing 118,000 lbs/day (53,524 kg/day) of solids to theanaerobic digester 490.

A comparison of the results of the calculations comparing the systemwith and without the DAF to contact tank solids recycle is illustratedin Table 1 below:

TABLE 1 System operated without System with 62% DAF → Contact DAF →Contact Parameter tank recycle tank recycle BOD treated in biological41,200 20,600 treatment tank (lbs/day) (18,688 kg/day) (9,344 kg/day)Aeration energy (both   600   410 contact tank and biological treatmenttank, kW) Solids to anaerobic 103,000 115,000 digester (lbs/day) (46,720kg/day) (52,163 kg/day) Solids destroyed (lbs/day) 43,900 55,900 (19,913kg/day) (25,356 kg/day) Biogas produced (mcfd/ 0.66 0.84 day) (18,633cubic (23,730 cubic meters/day) meters/day) Biogas energy (assuming1,880 2,390 40% conversion efficiency, kW) Net energy gain (kW) 1,2801,880

These results show that providing a wastewater treatment system asconfigured in FIG. 10 with a recycle of solids removed in a DAF unit toa contact tank can significantly reduce the energy required to operatethe system as compared to an equivalent system without the recycle ofsolids from the DAF unit to the contact tank. Adding the DAF to contacttank solids recycle results in less BOD being sent for treatment in thebiological treatment tank (a reduction of (41,200−20,600)/41,200=50% inthe present example) which lowers the need for aeration in thebiological contact tank. A greater amount of biogas((0.84−0.66)/0.66=27% more in the present example) is produced whenadding the DAF to contact tank solids recycle to the system. Thecombined gain in biogas production and decrease in aeration energyrequirements results in a net energy gain of 1,880−1,280=600 kW whenadding the DAF to contact tank solids recycle to the system. At anestimated $0.10/kW energy cost, this net energy gain would yield a costsavings of about $530,000 per year.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is:
 1. A wastewater treatment system comprising: a contact tank having a first inlet fluidly connectable to a source of wastewater to be treated, and an outlet, the contact tank configured to treat the wastewater with activated sludge to form a first mixed liquor; a dissolved air flotation unit having a first inlet fluidly connected to the outlet of the contact tank, a second inlet fluidly connected to a source of gas, a first floated solids outlet, a second floated solids outlet, and an effluent outlet, the dissolved air flotation unit configured to treat the first mixed liquor with the gas to form floated solids and an effluent; a fermentation unit having a first inlet fluidly connected to the first floated solids outlet, a first fermented solids outlet, and a second fermented solids outlet, the fermentation unit configured to treat at least a portion of the floated solids to form fermented solids; an anaerobic digester having a first inlet fluidly connected to the second floated solids outlet, a second inlet fluidly connected to the second fermented solids outlet, and an outlet; and a biological treatment unit having a first inlet fluidly connected to the effluent outlet, a second inlet fluidly connected to the first fermented solids outlet, and an outlet, the biological treatment unit configured to treat the effluent with at least a portion of the fermented solids to form a second mixed liquor.
 2. The system of claim 1, wherein the contact tank has a second inlet and the biological treatment unit has a third inlet, the system further comprising: a solids-liquid separation unit having an inlet fluidly connected to the outlet of the biological treatment unit, a solids-lean effluent outlet, a first return activated sludge outlet fluidly connected to the second inlet of the contact tank, and a second return activated sludge outlet fluidly connected to the third inlet of the biological treatment unit, the solids-liquid separation unit configured to treat solids from the second mixed liquor to form a solids-lean effluent and return activated sludge.
 3. The system of claim 1, further comprising a screen positioned between the first floated solids outlet and the first inlet of the fermentation unit, the screen configured to retain non-biological waste solids.
 4. The system of claim 1, further comprising: a first metering valve positioned upstream from the first fermented solids outlet and the second fermented solids outlet; and a first controller operatively connected to the first metering valve and configured to instruct the first metering valve to selectively dispense the fermented solids to the biological treatment unit and to the anaerobic digester.
 5. The system of claim 1, further comprising a screen positioned between the second floated solids outlet and the first inlet of the anaerobic digester, the screen configured to retain non-biological waste solids.
 6. The system of claim 1, wherein the fermentation unit further comprises a third fermented solids outlet and the contact tank further comprises a third inlet fluidly connected to the third fermented solids outlet.
 7. The system of claim 6, further comprising a screen positioned between the third fermented solids outlet and the third inlet of the contact tank, the screen configured to retain non-biological waste solids.
 8. The system of claim 1, wherein the contact tank comprises a second outlet and the biological treatment unit comprises: an anaerobic region having a first inlet fluidly connected to the effluent outlet, and an outlet; an aerated anoxic region having a first inlet fluidly connected to the second outlet of the contact tank, a second inlet fluidly connected to the outlet of the anaerobic region, and an outlet; and an aerobic region having a first inlet fluidly connected to the outlet of the aerated anoxic region, a second inlet fluidly connected to the effluent outlet, and an outlet.
 9. The system of claim 8, wherein at least one of the anaerobic region and the aerated anoxic region has an inlet fluidly connected to the first fermented solids outlet.
 10. The system of claim 9, wherein the anaerobic region has a second inlet fluidly connected to the first fermented solids outlet and the aerated anoxic region has a third inlet fluidly connected to the first fermented solids outlet, the system further comprising: a second metering valve positioned downstream from the first fermented solids outlet; and and a second controller operatively connected to the second metering valve and configured to instruct the second metering valve to selectively dispense the fermented solids to the anaerobic region and to the aerated anoxic region.
 11. The system of claim 8, wherein the biological treatment unit further comprises a post-anoxic region having a first inlet fluidly connected to the outlet of the aerobic region, a second inlet fluidly connected to the first fermented solids outlet, and an outlet.
 12. A method of treating wastewater comprising: directing the wastewater into a contact tank and mixing the wastewater with an activated sludge to form an activated mixed liquor; directing the activated mixed liquor into a dissolved air flotation unit and separating the activated mixed liquor to form a floated biosolids and an effluent; selectively directing a first portion of the floated biosolids into a fermentation unit and a second portion of the floated biosolids into an anaerobic digester, and fermenting the first portion of the floated biosolids in the fermentation unit to form a fermented solids; directing the effluent to a biological treatment unit; and selectively directing a first portion of the fermented solids into the biological treatment unit and a second portion of the fermented solids into the anaerobic digester, an amount of the first portion of the fermented solids directed into the biological treatment unit being selected responsive to a determination of a required soluble chemical oxygen demand of the biological treatment unit, and biologically treating the effluent with the first portion of the fermented solids in the biological treatment unit to form a biologically treated mixed liquor.
 13. The method of claim 12, further comprising: directing the biologically treated mixed liquor into a solids-liquid separation unit and separating the biologically treated mixed liquor to form a solids-lean effluent and the activated sludge; directing a first portion of the activated sludge into the contact tank; and directing a second portion of the activated sludge into the biological treatment unit.
 14. The method of claim 12, further comprising separating non-biological waste solids from at least one of the first portion of the floated biosolids and the second portion of the floated biosolids.
 15. The method of claim 12, further comprising directing a third portion of the fermented solids into the contact tank.
 16. The method of claim 15, further comprising separating non-biological waste solids from the third portion of the fermented solids.
 17. The method of claim 12, wherein the first portion of the fermented solids is selectively directed into at least one of an anaerobic region, an aerated anoxic region, and a post-anoxic region of the biological treatment unit.
 18. The method of claim 12, wherein treating the effluent with the first portion of the fermented solids comprises reducing a concentration of dissolved phosphorous in the effluent when forming the biologically treated mixed liquor.
 19. A wastewater treatment system comprising: a contact tank having a first inlet fluidly connectable to a source of wastewater to be treated, a first outlet, and a second outlet, the contact tank configured to treat the wastewater with activated sludge to form a first mixed liquor; a dissolved air flotation unit having a first inlet fluidly connected to the first outlet of the contact tank, a second inlet fluidly connected to a source of gas, a first floated solids outlet, and an effluent outlet, the dissolved air flotation unit configured to treat the first mixed liquor with the gas to form floated solids and an effluent; a fermentation unit having a first inlet fluidly connected to the first floated solids outlet, and a first fermented solids outlet, the fermentation unit configured to treat at least a portion of the floated solids to form fermented solids; and a biological treatment unit including: an anaerobic region having a first inlet fluidly connected to the effluent outlet, and an outlet; an aerated anoxic region having a first inlet fluidly connected to the second outlet of the contact tank, a second inlet fluidly connected to the outlet of the anaerobic region, and an outlet; an aerobic region having a first inlet fluidly connected to the outlet of the aerated anoxic region, a second inlet fluidly connected to the effluent outlet, and an outlet; a first inlet fluidly connected to the effluent outlet; a second inlet fluidly connected to the first fermented solids outlet; and an outlet, the biological treatment unit configured to treat the effluent with at least a portion of the fermented solids to form a second mixed liquor.
 20. A method of treating wastewater comprising: directing the wastewater into a contact tank and mixing the wastewater with an activated sludge to form an activated mixed liquor; directing the activated mixed liquor into a dissolved air flotation unit and separating the activated mixed liquor to form a floated biosolids and an effluent; selectively directing a first portion of the floated biosolids into a fermentation unit and a second portion of the floated biosolids into an anaerobic digester, and fermenting the first portion of the floated biosolids in the fermentation unit to form a fermented solids; directing the effluent to a biological treatment unit; and selectively directing a first portion of the fermented solids into the biological treatment unit and a second portion of the fermented solids into the anaerobic digester, and biologically treating the effluent with the first portion of the fermented solids in the biological treatment unit to form a biologically treated mixed liquor, treating the effluent with the first portion of the fermented solids comprising reducing a concentration of dissolved phosphorous in the effluent when forming the biologically treated mixed liquor. 